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HD Tune
We'll kick things off with HD Tune, which replaces HD Tach as our synthetic benchmark of choice. Although not necessarily representative of real-world workloads, HD Tune's targeted tests give us a glimpse of a drive's raw capabilities. From there, we can explore whether the drives live up to their potential.

The Momentus XT's read speeds are disappointingly sluggish. Not only does the hybrid fail to match the performance of the much cheaper 7200.4, but it also has a slower average read speed than the 5,400-RPM Scorpio Blue. The desktop drives and SSDs are way ahead in this test.

If you look up at the line graph, you'll see that the XT's performance drops from nearly 100MB/s to just 75MB/s across the first 20% of the drive. The 7200.4's read speed only falls to 94MB/s over the same interval.

HD Tune's write speed tests take the XT's flash memory out of the equation, which seems to suit the hybrid just fine. This time around, the new Momentus is a little bit faster than the 7200.4. The XT is even quicker than a couple of our SSD configs.

Unlike in the read speed tests, we don't see a precipitous drop in the XT's performance over the first portion of the drive. Writes never touch the flash, suggesting that the read-speed drop-off is tied to Adaptive Memory.

Next up: some burst-rate tests that should test the cache speed of each drive. We've omitted the X25-V RAID array from the following results because it uses a slice of system memory as a drive cache.

The Momentus XT appears to have a faster DRAM cache than the 7200.4. Seagate's hybrid offers quicker burst speeds than its purely mechanical forebear, although the XT isn't fast enough to catch the Scorpio Black.

Our HD Tune tests conclude with a look at random access times, which the app separates into 512-byte, 4KB, 64KB, and 1MB transfer sizes.

We didn't observe the Momentus XT's performance improve from one run to the next in HD Tune's sustained or burst speed tests. However, Adaptive Memory quickly, er, adapted to the app's access time tests. It only took a couple of runs for the XT's access times to stabilize, putting the drive within striking distance of our collection of SSDs with transfer sizes up to 64KB.

The 1MB transfer size raises the access times of our SSDs by an order of magnitude, and it's not kind to the Momentus XT, either. With this large transfer size, the XT finds itself near the back of the field with the mechanical notebook drives. How pedestrian.

With its flash memory exclusively dedicated to reads, the Momentus XT behaves like a traditional hard drive in HD Tune's write access time tests. Unfortunately, that means much slower access times than the SSDs. The XT also falls behind the Scorpio Black at the three smallest transfer sizes, although the hybrid stays a few steps ahead of the 7200.4. I expect the XT's larger cache deserves most of the credit on that front.