Seagate Momentus XT 500GB review

The Seagate Momentus XT is a hybrid hard disk, combining a standard 7200RPM 2.5in drive with 4GB of SLC flash memory to speed things up. At first glance, hybrid disks look like a good idea, but the last such device we saw – the SilverStone HDDBOOST – ended up wasting most of its fast solid-state cache on rarely-accessed files.

With the Momentus XT, Seagate has tried to take a more intelligent approach. The company’s “Adaptive Memory” system constantly monitors the files you use and automatically caches the most frequently accessed data in the SSD buffer for faster access. We presume there’s also some write buffering involved – otherwise the use of fast-writing SLC chips, rather than slower MLC memory, would seem odd.

Seagate claims that, in some synthetic benchmarks, the Momentus XT’s buffering gives it an 80% performance improvement over a regular 7,200RPM drive. That may be true, but buffering is always likely to assist repetitive tests more than a real-world multitasking workload.

Indeed, the actual benefit you'll see is very hard to quantify, because the drive’s adaptive process can cause wild variations in performance between two functionally identical operations. That was amply illustrated when we tried repeatedly copying 1.5GB test files from a RAM disk to the Momentus XT: most times the transfer took around 16 seconds, but it frequently dipped down to 10 seconds, or rose as high as 19 seconds. We’re used to seeing some variation between test runs, but a 90% gap between best and worst performance is unprecedented.

It was a similar story with our small file copy tests. Different copies of the same folder took between 37 and 59 seconds to copy, with no visible reason for the variation. This unpredictability played havoc with the Windows file copy dialogue too: several times we saw its “time remaining” readout flick from “about ten seconds” to “about two and a half minutes” and back.

Next to these findings, read performance was surprisingly consistent, with both the large file and small file tests varying by just a few per cent between five test runs – much more what we’re used to. Annoyingly, there’s no way to find out exactly how the drive is using its SSD cache, but these very uniform results suggest that the drive didn't buffer these test files, even though we accessed them five times in a row.

Taking average transfer rates across our tests, we come up with figures of 118MB/sec and 89MB/sec for large file writing and reading, and 28MB/sec and 58MB/sec respectively in the small file tests. These results are merely comparable to the regular 2.5in Toshiba drive we tried out in our last internal hard disk Labs test, and certainly don’t come close to the speeds attainable by a pure SSD such as our recommended Crucial M225.

Nor did the Momentus XT bring startup times down to SSD levels. Booting from a Crucial M225, our Core i7-920 test system reported an uptime of just 13 seconds when the desktop appeared. With Seagate’s drive the process took 20 seconds – just as we’d expect from a typical mechanical laptop drive.

Of course, it’s no mystery that Seagate’s hybrid offering can’t keep up with the Crucial drive: the M225 costs over £350 exc VAT, while the Momentus XT can be had for just £97. It’s a compromise candidate, and there’s nothing to be ashamed of in that.

But since the benefit you get is so uncertain and inconsistent, it’s impossible to recommend. For a desktop system you’ll do better with the Samsung Spinpont F3 - it's all mechanical, but it's faster than this hybrid drive and delivers a lot more capacity for your money.

Otherwise, if you need a 2.5in drive, we suggest you avoid the Momentus XT and stick with a regular 7200RPM one, which will cost you £30 less, and – in our experience – give very similar performance in everyday use.

Author: Darien Graham-Smith