For quite some time now, mechanical hard drives have stored data in 512-byte chunks called sectors. That sector size worked for lower capacity points, but as areal densities rise, it's become increasingly inappropriate for new drives. As a result, the industry has decided to transition to a 4KB sector size dubbed Advanced Format.
So-called legacy formatting schemes sandwich each 512-byte sector between Sync/DAM and ECC blocks that handle data address marking and error correction, respectively—and also take up space. You still need those blocks with Advanced Format, but only every 4KB rather than every 512 bytes, which translates to a dramatic reduction in overhead. This approach allows Advanced Format to make more efficient use of a platter's available capacity, and Western Digital expects it to boost useful storage by 7-11%, depending on the implementation. Current 500GB/platter products stand to see an increase in useful capacity of about 10%, which is really quite impressive.
Since not all operating systems can handle 4KB sectors natively, Western Digital's Adavanced Format implementation divides each 4KB physical sector into eight 512-byte logical sectors. The drive's firmware performs all the necessary translations, and according to Western Digital, there's no loss in performance as long as partitions are properly aligned. Windows 7 and Vista should align new partitions properly on their own, but you'll have to download a free WD Align application to align partitions correctly with Windows XP. WD Align is also necessary if you're using a disk cloning utility to create partitions under Vista or Win7.
Although Western Digital's Advanced Format implementation uses the drive's firmware to translate requests, users won't be able to add the feature to existing drives with a firmware upgrade. The drive's platters must be prepared for Advanced Format at the factory.
Western Digital is first rolling out Advanced Format in its Caviar Green line. 500GB drives featuring the new formatting scheme are scheduled to start shipping this week and should be followed quickly by higher capacity points. You'll be able to identify an Advanced-Format-compatible drive by its model number, WD10EARS, or by stickers on the drive and its packaging. These new models will also feature larger 64MB caches (previous Greens topped out at 32MB), although Western Digital doesn't expect their street prices to be any higher.