Conclusions
The Momentus XT wasn't quite what I was expecting from hybrid hard drive. Sure, it combines mechanical platters with flash memory, but Adaptive Memory is a much more one-sided approach than the old ReadyDrive hybrid scheme introduced with Windows Vista. Seagate's decision to use the flash exclusively as a read cache effectively eliminates any potential for improved write performance over mechanical hard drives. The company's focus on improving performance also means that the flash isn't used to conserve power.  In fact, the XT is likely to sap more battery life than mechanical hard drives because it must power a flash memory chip in addition to rotating media.

Adaptive Memory is a new approach for Seagate, a technology that's obviously been in development for some time given the vintage of the XT's platters. The fact that the hybrid scheme is completely independent of the operating system is easily its most impressive attribute, one that should bring a smile to the face of MacBook users who have held off on SSD purchases due to OS X's lack of TRIM support. But does it deliver on the performance front?

Sometimes, but not often. The Momentus XT showed flashes of potential in our random access time tests, offering SSD-like read access times for smaller transfer sizes. Adaptive Memory also cut Modern Warfare 2 load times nearly in half and shaved valuable seconds off Crysis: Warhead load times and the Windows 7 boot process. The XT fared extremely well in our file creation and copy tests, too, although that's more a credit to the drive's DRAM cache than its onboard flash.

While the Momentus XT offers good sequential write performance, sequential reads prove more problematic. The rapid drop in read speeds we observed in HD Tune isn't encouraging, and neither is the drive's poor read performance in FC-Test. Throw in a complete lack of performance scaling in IOMeter between 1 and 32 concurrent IO requests, and the XT looks to have just as many problems as it has potential.

Competition is the Momentus XT's biggest obstacle, though. A good solid-state drive is going to be faster across the board, and as our results illustrate, even better at the things that Adaptive Memory does well. The XT's relatively sluggish sequential read speeds also leave it vulnerable to mechanical notebook drives, particularly those with higher-density platters.

Given Adaptive Memory's limitations, I think you're better off with a two-drive hybrid that combines an SSD with secondary mechanical storage. Small-form-factor systems and larger gaming notebooks should be able to accommodate a second drive, and although such a configuration will set you back more than a single XT, I think it's definitely worthwhile for performance-minded enthusiasts.

If you're a notebook user with only a single hard drive bay, the Momentus XT starts to look more attractive, especially if that notebook is your primary PC. The only do-it-yourself hybrid option on that front would be an internal SSD with an external mechanical drive, which obviously has some pretty considerable drawbacks. A single Momentus XT is a more elegant and affordable solution. The Momentus XT slips into a single drive bay, and its 500GB of capacity will only set you back $130—just a few dollars more than a 40GB X25-V SSD that's barely large enough to house Windows 7 and a couple of games. Add in the quick load times that Adaptive Memory can facilitate and Seagate's five-year warranty, and the Momentus XT becomes a viable option for notebook users looking for a bit of a performance boost without compromising storage capacity or their budgets.