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Using the Select Target Drive item will bring up a standard Finder file dialog box, allowing you to locate the drive you wish to test. One problem you may run into: if you select the startup drive, you may see an error message that Disk Speed Test can’t be run on the selected drive because it is read only. This isn’t a bug, just a bit of a logistics problem. Disk Speed Test is run with the same user privileges as the login account you're using, and the app doesn't have the ability to ask to raise permission levels by asking you for your password.

The workaround is easy enough; when you wish to test your Mac’s startup drive, simply select your own home folder as the directory to be tested. You should then be able to run the speed tests without issues. Blackmagic refers to the test size as the stress size. It’s really just the size of the dummy file the app will use for writing and reading. The choices are 1 GB, 2 GB, 3 GB, 4 GB, and 5 GB. The size you choose is important; ideally, it needs to be larger than any cache a hard drive may include in its design. The idea is to make sure the Disk Speed Test is actually testing the write, read speed to the platters of a mechanical drive or the flash memory modules of an SSD, and not the faster memory cache used in the drive's controller.

Finally, if you're testing a Fusion drive, remember that a Fusion drive is usually not the best candidate for being the storage target for video recording or playback, since it's difficult to predict where the video files will be stored, on the fast SSD or the slow hard drive. Nevertheless, if you would like to measure the performance of your Fusion drive, use the larger 5 GB stress file size, and watch the speedometers closely. When you start the test, you'll likely see relatively slower write and read speeds as the first couple of tests are written to the slower hard drive. At some point, your will decide the test file is one you're using often, and move it to the faster SSD.

You can actually see this occur on the write and read speedometers.