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Seagate 500GB Momentus XT Hybrid Hard Drive

The Seagate 500GB Momentus XT is a hybrid drive, a conventional 500GB hard drive with 4GB of SSD flash memory tacked on for improved performance. At about $140, it’s considerably less expensive than a solid state drive (SSD) of similar capacity, though with SSD prices dropping steadily, that equation will continually change, and not everyone needs 500GB capacity.

I compare the Momentus XT to its siblings, the 750GB Momentus and the 500GB Momentus 7200.4.

The XT has a 32MB onboard cache also, whereas the other to models have a 16MB onboard cache, so the Momentus XT is definitely a higher-spec model, and it also carries a 5 year warranty. All of them are 7200 rpm drives.

To test real-world usage, I tried a variety of things that I do for my photographic work.

Application launch speed

The graph below is the approximate perceived overall responsiveness (time) it takes to launch a variety of applications with each drive. The differences vary significantly by application, so I opted for a relative merit graph that shows roughly how each drive feels.

The Seagate Momentus XT was particularly good at launching Adobe Photoshop, perhaps because it gets loaded into its onboard SSD cache. The other two Seagates definitely lagged when launching applications. However, the OWC Mercury Pro SSD feels almost instant, and is consistently fast with every application, whereas even the Seagate Momentus XT lags like any hard drive.

That said, it’s clear that the Momentus XT has notably better responsiveness than its two siblings, but remember, this is also on a nearly empty drive that has been used very little.

Some operations depend mostly on small reads and semi-random access and caching

Sustained transfer speed

Sustained transfer speed matters a great deal for some kinds of tasks, such as working with files in Photoshop that exceed memory limits (more on that further below), or saving or opening or copying large files. In such cases, the 4GB of onboard hybrid SSD cache on the Momentus carries no advantage, since the raw sustained speed is what matters.

Speed changes as the drive fills

The Seagate Momentus 750GB writes data notably faster than the Momentus XT 500GB, and thus for any given amount of stored data, it will maintain a higher speed, since hard drives slow down as they fill up.

A 750GB drive with 490GB of stored data will perform much better than a 500GB drive with 490GB stored. The graph shows that this speed advantage begins to grow as early as the 150GB mark (~300 offset).

Remember, if you store 250GB of data on a 500GB drive, anything additional that’s added will perform as shown from the 500-1000 mark below— slow. The speed loss accelerates greatly after the drive is half full.

The graph below shows what happens to sustained transfer speed as 500GB of data is written to and read on both drives. It’s an apples-to-apples comparison for real world usage with a fixed amount of data, highly appropriate and relevant to anyone having between 200GB and 500GB of data to store.


Tested with DiskTester, using the DiskTester fill-volume command.

Click for a larger graph. The heavier red/green lines are the Momentus XT write/read speeds. The thinner orange/blue lines are the Seagate 750GB. The black/gray lines are the Hitachi 7K500. Note the nice write/read consistency with the Hitachi.

Drive speed declines as the drive fills up
(riting 500GB of data as 1000 large files to fill the volume)

Photoshop CS5

This test is disk intensive, and correlates strongly with sustained transfer speed.

The single OWC Mercury Extreme Pro SSD shows a major advantage, but if anything this is understated for disk-related tasks in general.

Note that the Hitachi 7K500 is easily faster than either Seage offering, showing that real-world results are sometimes different than benchmarks or marketing claims would suggest.

Disk-intensive activity is mainly a function of sustained transfer speed


The 500GB Seagate Momentus XT offers some clear performance advantages for all-around tasks, but can’t do much of anything for tasks that require high sustained data transfer rates. Still, it does perfectly well at that task, and thus appears to be a very solid and low cost choice for a high performance drive. At about $140, it’s an easy choice for an upgrade should the budget not allow for a solid state drive.

However, if you intend to store 250GB or more on the drive, the Seagate Momentus 750GB is likely to be a substantially better performer for many purposes, since its speed will hold up longer for the same amount of data.

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