The portable SSD market has been steadily expanding thanks to the increasing digital footprint of consumers. Content creators find themselves capturing photos and videos at higher and higher resolutions and bit-rates, while gamers are encountering installation sizes running into 100s of GBs. And the backup requirements for the average consumer have also grown in tandem.

Meanwhile, technological advancements such as 3D NAND with high layer counts and the emergence of QLC have enabled SSD capacities to increase substantially over the last few years. With economies of scale kicking in, multiple vendors are finally able to offer consumer-focused flash-based storage devices in capacities up to 4TB. At the 2021 CES, Western Digital introduced 4TB variants of almost all drives in their portable SSD families, including their flagship SanDisk Extreme PRO v2 and the WD_BLACK P50 lines. In March, Crucial updated its affordable X6 lineup to include a 4TB version.

WD's flagships and Crucial's mainstream X6 offerings represent two ends of the pricing spectrum. At the same capacity point, they present an interesting view of the tradeoffs involved in achieving a particular price point - performance, consistency, BOM features, and value additions. This review looks at the features of the SanDisk Extreme PRO v2 4TB and the Crucial X6 4TB portable SSDs, along with an analysis of their performance numbers and value propositions.

Introduction and Product Impressions

External bus-powered storage devices have grown both in storage capacity as well as speeds over the last decade. While PCIe / NVMe has been the driver of SSD speeds, faster host interfaces (such as Thunderbolt 3 and USB 3.x) have helped on the direct-attached storage (DAS) side. We now have palm-sized flash-based storage devices capable of delivering 3GBps+ speeds. While those speeds can be achieved with Thunderbolt 4, mass-market devices have to rely on USB. Within the USB ecosystem, USB 3.2 Gen 2 (10 Gbps) is fast becoming the entry level for thumb drives and portable SSDs. Premium devices sporting the USB 3.2 Gen 2x2 (20 Gbps) interface have been making it to the market over the last 18 months or so, but have been held back by the lack of widespread host support in desktops and other computing platforms. In contrast, USB 3.2 Gen 2 (10 Gbps) has gained widespread deployment in both Type-C and Type-A versions.

The SanDisk Extreme PRO Portable SSD v2 4TB we are looking at in this review uses the same mainboard with the ASMedia ASM2364 brdige chip as the 2TB version reviewed in October 2020. The main update is in the capacity of the SN730E NVMe SSD used internally. Both capacities use the same 3D TLC NAND, with the increased capacities being enabled by flash packages on both sides of the M.2 SSD. It made sense for SanDisk to delay the launch of the 4TB version - waiting for the market demand to catch up, while also taking advantage of increased yields with the mature BiCS 4 fabrication process. In terms of package contents, IP rating, and the look and feel of the drive, there is no change from the version reviewed earlier.

Crucial has a long history of reusing product model names despite significant changes in the internals. The Crucial X6 Portable SSD 4TB also falls in the same category. We had reviewed the 2TB version last year with Crucial's QLC-based BX500 SATA SSD behind an ASMedia ASM235CM SATA-to-USB bridge chip. Crucial's X6 4TB is one of the first high-performance external SSDs to hit the market with a native USB flash controller. Essentially, it adopts a platform similar to the ones found on USB thumb drives. However, thanks to recent advancements, such controllers are now able to hit transfer rates up to 1.9GBps when configured with the appropriate NAND and upstream interface. The X6 4TB version uses Crucial's 96L 3D QLC with the Phison U17 controller. This combination enables Crucial to claim speeds of up to 800 MBps. Based on consumer reports, it appears likely that Crucial is replacing the SATA SSD / bridge chip configuration in the lower capacity X6 SKUs with the native flash controller based board.

A quick overview of the internal capabilities of the internal drives is given by CrystalDiskInfo.

Comparative Portable SSDs Configuration
Aspect	SanDisk Extreme PRO v2 4TBCrucial X6 Portable SSD 4TB	Crucial X6 Portable SSD 4TBSanDisk Extreme PRO v2 4TB
Downstream Port	1x PCIe 3.0 x4 (M.2 NVMe)	Native Flash
Upstream Port	USB 3.2 Gen 2x2 Type-C	USB 3.2 Gen 2 Type-C
Bridge Chip	ASMedia ASM2364	Phison U17
Power	Bus Powered	Bus Powered
Use Case	Premium IP55-rated 2GBps-class, compact, and sturdy portable SSD in a gumstick form-factor	Affordable 800MBps, compact, and durable portable SSD in a pocketable form-factor
Physical Dimensions	110.26 mm x 57.34 mm x 10.22 mm	69 mm x 64 mm x 11 mm
Weight	90 grams (without cable)	41 grams (without cable)
Cable	30 cm USB 3.2 Gen 2x2 Type-C to Type-C 30 cm USB 3.2 Gen 2 Type-C to Type-A	24 cm USB 3.2 Gen 2 Type-C to Type-C Type-C to Type-A adapter sold separately
S.M.A.R.T Passthrough	Yes	Yes
UASP Support	Yes	Yes
TRIM Passthrough	Yes	Yes
Hardware Encryption	Yes (256-bit AES, only via SanDisk Secure App)	Not Available
Evaluated Storage	Western Digital SN730E PCIe 3.0 x4 M.2 2280 NVMe SSD SanDisk / Toshiba BiCS 4 96L 3D TLC	Micron 96L 3D QLC
Price	USD 900	USD 481
Review Link	SanDisk Extreme PRO Portable SSD v2 4TB Review	Crucial X6 Portable SSD 4TB Review

Prior to looking at the benchmark numbers, power consumption, and thermal solution effectiveness, a description of the testbed setup and evaluation methodology is provided.

Testbed Setup and Evaluation Methodology

The evaluation routine for direct-attached storage devices – portable SSDs, storage bridges (including RAID enclosures), and memory cards – all utilize the same testbed and have similar workloads with slight tweaks based on the end market for the product. Our testbeds have kept pace with the introduction of new external interfaces - Thunderbolt 2, Thunderbolt 3, and USB 3.2 Gen 2 via Type-C. In mid-2014, we prepared a custom desktop based on Haswell, which was then upgraded to Skylake in early 2016. A botched Thunderbolt 3 firmware upgrade on the Skylake machine meant that we had to shift to the Hades Canyon NUC starting in early 2019. This year, we have adopted the Quartz Canyon NUC (essentially, the Xeon / ECC version of the Ghost Canyon NUC) along with build components from ADATA Industrial - 2x 16GB DDR4-3200 ECC SODIMMs and a PCIe 3.0 x4 NVMe SSD - the IM2P33E8 1TB.

The most attractive aspect of the Quartz Canyon NUC is the presence of two PCIe slots (electrically, x16 and x4) for add-in cards. In the absence of a discrete GPU - for which there is no need in a DAS testbed - both slots are available. In fact, we also added a spare SanDisk Extreme PRO M.2 NVMe SSD to the CPU direct-attached M.2 22110 slot in the baseboard in order to avoid DMI bottlenecks when evaluating Thunderbolt 3 devices. This still allows for two add-in cards operating at x8 (x16 electrical) and x4 (x4 electrical). SilverStone's SST-ECU06 USB 3.2 Gen 2x2 (20 Gbps) add-in card (based on the ASMedia ASM3242 host controller) was installed in the x4 slot. The Type-C port from this add-in card is used to evaluate all storage devices with a USB 3.x interface.

The specifications of the testbed are summarized in the table below:

AnandTech DAS Testbed Configuration
System	Intel Quartz Canyon NUC9vXQNX
CPU	Intel Xeon E-2286M
Memory	ADATA Industrial AD4B3200716G22 32 GB (2x 16GB) DDR4-3200 ECC @ 22-22-22-52
OS Drive	ADATA Industrial IM2P33E8 NVMe 1TB
Secondary Drive	SanDisk Extreme PRO M.2 NVMe 3D SSD 1TB
Add-on Card	SilverStone SST-ECU06 USB 3.2 Gen 2x2 Type-C Host
OS	Windows 10 Enterprise x64 (21H1)
Thanks to ADATA, Intel, and SilverStone for the build components

The evaluation scheme for DAS units involves multiple workloads which are described in detail in the corresponding sections.

• Synthetic workloads using CrystalDiskMark and ATTO
• Real-world access traces using PCMark 10's storage benchmark
• Custom robocopy workloads reflective of typical DAS usage
• Sequential write stress test

While most DAS units targeting a particular market segment advertise similar performance numbers and also meet them for common workloads, the real differentiation is brought out on the technical side by the performance consistency metric and the effectiveness of the thermal solution. Industrial design and value-added features may also be important for certain users. The remaining sections in this review tackle all of these aspects.

Synthetic Benchmarks - ATTO and CrystalDiskMark

Benchmarks such as ATTO and CrystalDiskMark help provide a quick look at the performance of the direct-attached storage device. The results translate to the instantaneous performance numbers that consumers can expect for specific workloads, but do not account for changes in behavior when the unit is subject to long-term conditioning and/or thermal throttling. Yet another use of these synthetic benchmarks is the ability to gather information regarding support for specific storage device features that affect performance.

Western Digital claims claims read and write speeds of around 2 GBps for the SanDisk Extreme PRO Portable SSD v2, while Crucial claims speeds of 800 MBps for the X6. Both of these are backed up by the ATTO benchmarks provided below. ATTO benchmarking is restricted to a single configuration in terms of queue depth, and is only representative of a small sub-set of real-world workloads. It does allow the visualization of change in transfer rates as the I/O size changes, with optimal performance being reached around 512 KB for a queue depth of 4 in the SanDisk model, and around 128KB for the X6.

CrystalDiskMark Benchmarks
TOP: SanDisk Extreme PRO v2 4TBCrucial X6 Portable SSD 4TB	BOTTOM: Crucial X6 Portable SSD 4TBSanDisk Extreme PRO v2 4TB

The ability of the SanDisk Extreme PRO to support the NCQ / UASP features was never in question. The X6 with its native flash controller is the more interesting product in this case, and we see that it does support the features essential for SSD-like performance.

AnandTech DAS Suite - Benchmarking for Performance Consistency

Our testing methodology for storage bridges / direct-attached storage units takes into consideration the usual use-case for such devices. The most common usage scenario is the transfer of large amounts of photos and videos to and from the unit. Other usage scenarios include the use of the unit as a download or install location for games and importing files directly from it into a multimedia editing program such as Adobe Photoshop. Some users may even opt to boot an OS off an external storage device.

The AnandTech DAS Suite tackles the first use-case. The evaluation involves processing five different workloads:

• AV: Multimedia content with audio and video files totalling 24.03 GB over 1263 files in 109 sub-folders
• Home: Photos and document files totalling 18.86 GB over 7627 files in 382 sub-folders
• BR: Blu-ray folder structure totalling 23.09 GB over 111 files in 10 sub-folders
• ISOs: OS installation files (ISOs) totalling 28.61 GB over 4 files in one folder
• Disk-to-Disk: Addition of 223.32 GB spread over 171 files in 29 sub-folders to the above four workloads (total of 317.91 GB over 9176 files in 535 sub-folders)

Except for the 'Disk-to-Disk' workload, each data set is first placed in a 29GB RAM drive, and a robocopy command is issue to transfer it to the external storage unit (formatted in exFAT for flash-based units, and NTFS for HDD-based units).

robocopy /NP /MIR /NFL /J /NDL /MT:32 $SRC_PATH $DEST_PATH

Upon completion of the transfer (write test), the contents from the unit are read back into the RAM drive (read test) after a 10 second idling interval. This process is repeated three times for each workload. Read and write speeds, as well as the time taken to complete each pass are recorded. Whenever possible, the temperature of the external storage device is recorded during the idling intervals. Bandwidth for each data set is computed as the average of all three passes.

The 'Disk-to-Disk' workload involves a similar process, but with one iteration only. The data is copied to the external unit from the CPU-attached NVMe drive, and then copied back to the internal drive. It does include more amount of continuous data transfer in a single direction, as data that doesn't fit in the RAM drive is also part of the workload set.

AnandTech DAS Suite - Performance Consistency
TOP: SanDisk Extreme PRO v2 4TBCrucial X6 Portable SSD 4TB	BOTTOM: Crucial X6 Portable SSD 4TBSanDisk Extreme PRO v2 4TB

The first three sets of writes and reads correspond to the AV suite. A small gap (for the transfer of the video suite from the internal SSD to the RAM drive) is followed by three sets for the Home suite. Another small RAM-drive transfer gap is followed by three sets for the Blu-ray folder. This is followed up with the large-sized ISO files set. Finally, we have the single disk-to-disk transfer set. The three write sets for each component are remarkably consistent across both SSDs. The controller behavior in the X6 for the disk-to-disk write case is interesting to analyze - With 32 parallel threads copying data from different folders, it appears that the controller gets a bit overwhelmed, hitting only around 550 MBps, before recovering and stepping up to around the expected 800 MBps mark. The SanDisk Extreme PRO v2, on the other hand, handles the 32-thread copy well and drops down only as the host system tries to catch up and send across more small-sized files. Once past those dips, the performance numbers stay at around 1.6 GBps for the rest of the write workload. On the temperature front, both units behave similarly, though the final temperature of the SanDisk unit is worth appreciating - despite the higher performance, the peak of 61C was lower than the X6's 62C.

PCMark 10 Storage Bench - Real-World Access Traces

There are a number of storage benchmarks that can subject a device to artificial access traces by varying the mix of reads and writes, the access block sizes, and the queue depth / number of outstanding data requests. We saw results from two popular ones - ATTO, and CrystalDiskMark - in a previous section. More serious benchmarks, however, actually replicate access traces from real-world workloads to determine the suitability of a particular device for a particular workload. Real-world access traces may be used for simulating the behavior of computing activities that are limited by storage performance. Examples include booting an operating system or loading a particular game from the disk.

PCMark 10's storage bench (introduced in v2.1.2153) includes four storage benchmarks that use relevant real-world traces from popular applications and common tasks to fully test the performance of the latest modern drives:

• The Full System Drive Benchmark uses a wide-ranging set of real-world traces from popular applications and common tasks to fully test the performance of the fastest modern drives. It involves a total of 204 GB of write traffic.
• The Quick System Drive Benchmark is a shorter test with a smaller set of less demanding real-world traces. It subjects the device to 23 GB of writes.
• The Data Drive Benchmark is designed to test drives that are used for storing files rather than applications. These typically include NAS drives, USB sticks, memory cards, and other external storage devices. The device is subjected to 15 GB of writes.
• The Drive Performance Consistency Test is a long-running and extremely demanding test with a heavy, continuous load for expert users. In-depth reporting shows how the performance of the drive varies under different conditions. This writes more than 23 TB of data to the drive.

Despite the data drive benchmark appearing most suitable for testing direct-attached storage, we opt to run the full system drive benchmark as part of our evaluation flow. Many of us use portable flash drives as boot drives and storage for Steam games. These types of use-cases are addressed only in the full system drive benchmark.

The Full System Drive Benchmark comprises of 23 different traces. For the purpose of presenting results, we classify them under five different categories:

• Boot: Replay of storage access trace recorded while booting Windows 10
• Creative: Replay of storage access traces recorded during the start up and usage of Adobe applications such as Acrobat, After Effects, Illustrator, Premiere Pro, Lightroom, and Photoshop.
• Office: Replay of storage access traces recorded during the usage of Microsoft Office applications such as Excel and Powerpoint.
• Gaming: Replay of storage access traces recorded during the start up of games such as Battlefield V, Call of Duty Black Ops 4, and Overwatch.
• File Transfers: Replay of storage access traces (Write-Only, Read-Write, and Read-Only) recorded during the transfer of data such as ISOs and photographs.

PCMark 10 also generates an overall score, bandwidth, and average latency number for quick comparison of different drives. The sub-sections in the rest of the page reference the access traces specified in the PCMark 10 Technical Guide.

Booting Windows 10

The read-write bandwidth recorded for each drive in the boo access trace is presented below.

Full System Drive Benchmark Bandwidth (MBps)Full System Drive Benchmark Latency (us)Full System Drive Benchmark ScoreExpand All

The Extreme PRO v2's score of 1345 for the 4TB version is miles ahead of the 1284 obtained for the 2TB SKU with the old testbed. The X6's 586 puts it almost in the same league as the Sabrent Rocket Nano Rugged 2TB which uses a M.2 2242 NVMe SSD.

Overall, we see the Extreme PRO v2 and X6 being leaders in their own performance class.

Worst-Case Performance Consistency

The performance of the storage bridges / drives in various real-world access traces as well as synthetic workloads was brought out in the preceding sections. We also looked at the performance consistency for these cases. Power users may also be interested in performance consistency under worst-case conditions.

Flash-based storage devices tend to slow down in unpredictable ways when subject to a large number of small-sized random writes. Many benchmarks use that scheme to pre-condition devices prior to the actual testing in order to get a worst-case representative number. Fortunately, such workloads are uncommon for direct-attached storage devices, where workloads are largely sequential in nature. Use of SLC caching as well as firmware caps to prevent overheating may cause drop in write speeds when a flash-based DAS device is subject to sustained sequential writes.

Our Sequential Writes Performance Consistency Test configures the device as a raw physical disk (after deleting configured volumes). A fio workload is set up to write sequential data to the raw drive with a block size of 128K and iodepth of 32 to cover 90% of the drive capacity. The internal temperature is recorded at either end of the workload, while the instantaneous write data rate and cumulative total write data amount are recorded at 1-second intervals.

Sequential Writes to 90% Capacity - Performance Consistency
TOP: SanDisk Extreme PRO v2 4TBCrucial X6 Portable SSD 4TB	BOTTOM: Crucial X6 Portable SSD 4TBSanDisk Extreme PRO v2 4TB

The SanDisk Extreme PRO v2 4TB maintains peak performance of around 1850 MBps for around 30 seconds before moving down a bit to a respectable 1600 MBps for the rest of the workload. The numbers point to a SLC cache of around 60GB, but the direct TLC write has very goood performance too. The temperature delta between the start and the end was 15C, but the maximum temperature was only 56C pointing to a very effective thermal solution.

The Crucial X6 maintains peak advertised performance of around 780 MBps for more than 16 minutes, pointing to a SLC cache of around 787 GB. However, once the cache runs out, the speeds move down to around 75 MBps till around 1.47TB of total continuous data writes. Beyond that, the speeds go down to around 50 MBps for the rest of the workload. The temperature delta is around 61C, but the slow writes in the latter part of the process means that not much heat gets generated anyway. The thermal solution with the plastic casing for the SSD is acceptable.

Power Consumption and Concluding Remarks

Bus-powered devices can configure themselves to operate within the power delivery constraints of the host port. While Thunderbolt ports are guaranteed to supply up to 15W for client devices, USB 2.0 ports are guaranteed to deliver only 2.5W (500mA @ 5V). In this context, it is interesting to have a fine-grained look at the power consumption profile of the various external drives. Using the Plugable USBC-TKEY, the bus power consumption of the drives was tracked while processing the CrystalDiskMark workloads (separated by 5s intervals). The graphs below plot the instantaneous bus power consumption against time, while singling out the maximum and minimum power consumption numbers.

CrystalDiskMark Workloads - Power Consumption
TOP: SanDisk Extreme PRO v2 4TBCrucial X6 Portable SSD 4TB	BOTTOM: Crucial X6 Portable SSD 4TBSanDisk Extreme PRO v2 4TB

The Gen 2x2 Extreme PRO v2 has a peak power consumption of around 7.23W, and idles at around 3W. It does go to sleep after around 20 minutes, dropping down to 0.91W. The X6 on the other hand has a peak power consumption of 2.95W. The idling power is around 0.75W. The drive didn't go to a lower power state below the 0.75W mode - in fact, after some idling time, it appears that some sort of garbage collection / moving of data from SLC to QLC takes place that causes significant power spikes.

Final Words

The preceding sections took a detailed look at two 4TB external SSDs - the SanDisk Extreme PRO Portable SSD v2 focusing on no-holds barred performance, and the Crucial X6 Portable SSD targeting the mainstream market with affordability as its primary focus.

Western Digital/SanDisk has taken the highest-performing USB bridge chip in the market, and coupled that with their highest performance SSD sporting full compatibility with that bridge chip. One could argue that a Thunderbolt 3 SSD would exhibit better performance, but the counter for that involves two different aspects - getting a Thunderbolt 3 SSD to work with USB hosts is necessary for wider compatibility, particularly for the Extreme PRO v2's target market. This involves integrating a USB bridge chip as well as a separate Thunderbolt 3 device controller on the board, increasing BOM cost and complexity. Additionally, Windows appears to treat Thunderbolt 3 SSDs as internal PCIe SSDs - when coupled with the default write caching disabled state, the write performance of Thunderbolt 3 SSDs becomes abysmal compared to even USB 3.2 Gen 2 (10 Gbps) SSDs. As seen in the benchmark numbers, the Extreme PRO v2 handily surpasses all Thunderbolt 3 SSDs in write-intensive workloads.

Crucial has gone in for aggressive optimizations in terms of platform cost. The product comes with a single Type-C to Type-C cable, and the Type-C to Type-A adapter is a separate purchase. Almost all modern PCs come with Type-C ports, so this might not be a concern for most consumers, but it does allow Crucial to push the pricing down further for the base SKU. The company has also not pursued any IP-rating for the device (the Extreme PRO v2 is IP55-rated). And unlike the Extreme PRO v2, the X6 doesn't feature hardware encryption. While the previous X6 had flash chips, a SATA SSD controller, and a bridge chip, the 4TB SKU comes with the flash chips and a native UFD (USB flash drive) controller. Coupling that with the use of QLC NAND (compared to Western Digital's 3D TLC in the Extreme PRO v2) means that the X6 offers the same storage capacity at a price point that is around half that of the SanDisk Extreme PRO v2.

Both SSDs support TRIM and S.M.A.R.T pass through, which are essential for keeping the drive healthy and monitoring it. Content creators and power users will definitely appreciate the SanDisk Extreme PRO v2 for its combination of features and performance. That great performance does come at a price, however, and after launching at $750 back in January, the drive now retails for $900. Despite the premium, it is likely that there is a significant market for the drive given how little competition there is within its performance class. The only downside for Western Digital is that USB 3.2 Gen 2x2 (20 Gbps) is yet to gain widespread traction, but that seems to be slowly changing.

Casual users will find the Crucial X6 at $480 fitting their requirements quite well. Though based on QLC NAND, with almost 800GB of SLC cache, most users are unlikely to run into the abysmal 75 MBps and 50 MBps writes. The drive also seems to be aggressive about moving data from the SLC buffer to its final place within the QLC NAND, and hence it is bound to regain performance quite quickly. At lower capacities, QLC could be quite problematic even for mainstream use-cases, but at 4TB – and with almost 20% of the capacity configured as SLC – it shouldn't matter. If better performance is desired, one has to be ready to fork out more – Western Digital has a host of 4TB 3D TLC options ranging from $680 to $900.