It's not just planar NAND that's running into physical limits lately. According to Seagate, its latest 1TB platter 3.5" drives have shrunk read/write heads as small as they can physically go. Similarly, tracks on those platters are placed as close together as physically possible. Pushing areal density is important to increase overall capacities (no one wants to see more platters per drive), but if we're at physical limits today then it's time for some architectural changes to push capacities going forward.

Seagate's solution is something it calls Shingled Magneting Recording (SMR). The process is pretty simple. Track size is traditionally defined by the size of the write heads, as they are larger than the read heads. The track width is larger than necessary from the perspective of reading data back in order to decrease the chances of reading data from adjacent tracks. Seagate's SMR exploits this reality.

SMR shrinks the guard space between tracks and allows tracks to overlap one another, like roofing shingles. Although data is written to the entire width of the track, a smaller/trimmed portion of the track (the width of the read head) is all that the drive cares about. By allowing tracks to overlap, areal density can continue to scale without further shrinking the size of the heads. 

The obvious downside of SMR is actually very NAND flash-like. When writing data sequentially to an empty platter, SMR is full of advantages. When you're writing to a series of tracks that already contains data, the SMR writing process is actually destructive. Since the writer remains full width and tracks now overlap, overwriting one track will actually harm the next track; those subsequent tracks will need to be overwritten as a result. 

Seagate's SMR groups tracks into bands, with the end of each band breaking the shingled track layout. Breaking the shingled layout regularly reduces max attainable density, but it makes it so that overwriting a portion of one track doesn't force a re-write of the complete disk. At worst, overwriting some sectors will force a re-write of an entire band, not an entire platter.

Seagate claims it has already shipped 1 million SMR enabled drives (I didn't actually know any SMR drives had been shipping), but plans on using the technology to increase areal densities beginning next year. In 2014 Seagate will move from a 1TB per platter design to 1.25TB per platter thanks to SMR. The increase in platter density will allow Seagate to ship a 4 platter/5TB drive next year. Seagate is hoping to hit higher densities without any performance degradation compared to existing SMR designs. The real question is whether or not Seagate can maintain similar full drive performance compared to a non-SMR drive.