Hybrid hard drives (H-HDDs) combine a hard drive and some non-volatile memory in a single unit. They will be available from Samsung in early 2007 (and perhaps from Seagate too). Much of the coverage in the computing press associates them with Windows Vista: Vista Premium (the highest-end version of the OS) makes H-HDDs a logo requirement for mobile systems. Vista includes special support for H-HDDs, with the marketing name ReadyDrive, suggesting that other operating systems will not be able to take full advantage of these devices.

So are H-HDDs just a marketing gimmick? They certainly seem to have commercial advantages for Microsoft and Samsung independent of any technical issues. Microsoft gets a new gimmick that only works fully with Vista, providing an extra incentive for people to migrate to the new operating system. Samsung gets to increase its share of the hard drive market and its margins on hard drives. It is the leading producer of non-volatile NAND memory, so it has an advantage in the market for these devices over other hard drive manufacturers. And even if/when other manufacturers start producing them, the extra demand for non-volatile memory will benefit Samsung.

So are there any real technical advantages? Well, there is a Microsoft WinHEC 2006 presentation on this page which gives some details. The most interesting points for me:

• A key advantage is that the drive can satisfy writes while the platters are spun down to conserver power, by buffering into its NV memory. Note that this requires the NV memory to be physically attached to the drive, to avoid the risk of losing those writes. This distinguishes H-HDDs from the ReadyBoost techonology in Vista: ReadyBoost only acts as a cache, not as a write buffer, because ReadyBoost is aimed at removable USB Flash drives.
• The presentation discusses the benefits for boot and wake-up times, though it doesn't say how this compares to reorganizing or prefetching the relevant blocks on the disk (as the Prefetcher in Windows XP and BootCache in Mac OS X do; have they already taken that as far as it will go?). But there is one definite advantage to H-HDDs here: The OS can start booting/waking up before the platters are up to speed, saving 2-3 seconds.
• It's not clear from the presentation how much of the management of the NV memory is done by the drive firmware, and how much by Vista. It does say that Vista talks to the drive in order to pin certain blocks into the NV memory, to accelarate the boot process. But I assume that the drive firmware can take advantage of the NV memory without OS assistance, in order to work well with XP.