External Drive Speed


Today’s Question: You recommended only using a very fast hard drive if you are going to store your Lightroom catalog on an external drive. But how do you determine if a particular hard drive is fast?

Tim’s Quick Answer: You want to ignore the maximum theoretical speed you might see in the promotional materials for a given hard drive, and instead try to determine the sustained data transfer rate the drive is actually capable of, the maximum speed of the interface used to connect the drive to your computer, and the speed capabilities of your computer with regard to a connected hard drive. In general I would suggest choosing an SSD (solid state drive) device that connects with the fastest data connection available on your computer.

More Detail: Unfortunately, it isn’t generally very easy to determine the actually data transfer speed capabilities of a hard drive. The best solution (which isn’t very practical) would be to perform actual data transfer tests on a wide range of hard drives. However, such tests aren’t widely performed and published in a way that would make it easy to choose a particular drive.

In some cases, the speed rating I see on packaging and in advertisements for hard drives reflects the theoretical maximum speed of the interface connecting the drive to the computer. In the case of USB 3.0, for example, you might see an indication that the drive supports up to 600 MB per second, when in fact a more realistic expectation for an external hard drive connected via USB 3.0 would be on the order of 100 to 200 MB per second.

This misleading advertising is (thankfully) not as common as it used to be. Instead most hard drive manufacturers seem to be simply leaving sustained transfer rates out of the specifications, and instead only telling you that the drive supports, for example, USB 3.0. To me this is just a clever way of leading you to believe that the drive will transfer data as fast as USB 3.0 will allow, which isn’t true in most cases.

When you can find an indication of the sustained data transfer speed (such as through online tests and reviews), that can be very helpful information. Otherwise I would opt for an SSD (solid state drive) storage device over a “traditional” hard drive with spinning platters, and I would opt for the fastest data connection you are able to use with your computer. With a traditional hard drive there is also a general correlation between the speed at which the platters spin (such as 5400 or 7200 rpm), so when you have the choice I would opt for the faster rotational speed.

But ultimately, even with the same specifications, a variety of different hard drives will be capable of transferring data at different rates. You can help tip the odds in your favor by opting for the fastest connection your computer supports and opting for an SSD drive. But whenever possible it is best to see actual performance testing results so you have a better sense of the performance you can expect.