Blackmagic Disk Speed Test: How Fast Are Your Mac's Drives?

Just how fast is that new drive you hooked up to your Mac? Blackmagic Disk Speed Test is one of the free disk benchmarking tools available for your Mac that can give you the lowdown on your Mac's disk speed.

What Is Blackmagic Disk Speed Test?

If you tried to find out the speed rating of a disk by checking a manufacturer's website, you might have stumbled through marketing materials that contained performance numbers with no context. That's one reason to use the Blackmagic Disk Speed Test to evaluate a Mac's performance, including how well the internal or external storage drives perform.

Blackmagic Disk Speed Test started as a free utility included with any of the Blackmagic Design video and audio products for multimedia capture, playback, and editing. The free app became popular with Mac enthusiasts as an easy way to check the performance of their system drives, fusion drives, and SSDs. While Blackmagic makes the app freely available to anyone, its emphasis is on video capture and playback.

How to Get the Blackmagic Disk Speed Test

Blackmagic released the app to the public via the Mac App Store, so visit the Mac App Store to download and install the Blackmagic Disk Speed Test.

How to Run the Blackmagic Disk Speed Test

Here's what to do when you're ready to test a Mac's internal or external drive:

1. Launch the Blackmagic Disk Speed Test from the Applications folder or by using the Launchpad on the Dock.

2. Select Settings (gear icon) above the Start button.

3. Click Select Target Drive from the drop-down menu.

4. In the left panel of the Finder screen, choose the disk or Mac volume you want to test, then click Open.

5. Click the Settings button, then choose the size of the Stress file the application will use. Options range from 1 GB to 5 GB.

6. Click the Speed Test Start button and watch as the information loads on the screen. 

How to Read the Results

Below the two main speedometers are the Will It Work and How Fast results panels. The Will It Work panel includes a list of common video formats, ranging from simple PAL and NTSC on up to 2K formats. Each format in the panel has multiple options for color bit depths, and individual read or write checkboxes. As a test runs, the panel fills with green checkmarks for each format, depth, and read and write speed that the volume under test can support for video capture and playback.

The How Fast panel works the same way, but instead of checkboxes, it displays the write and read frame rates that the drive under test can support for each of the formats.

Test Size Options

Blackmagic refers to the test size as the stress size. It's the size of the dummy file the app uses 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 tests the write and read speeds 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.

When testing the performance of a modern drive, use the 5 GB stress size. In addition, let the test run through more than one write and read cycle. When testing an SSD, use the smallest stress size, since you aren't as worried about an onboard cache.

How to Test a Fusion Drive

When testing a fusion drive, it's difficult to predict where the video files will be stored, on the fast SSD or the slow hard drive. Nevertheless, if you want 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 Mac decides the test file is one you use often and moves it to the faster SSD. You can see this occur on the write and read speedometers.

The Actual Test

The test starts by writing the test file to the target disk and then reading the test file back. The actual time spent writing is limited to an 8-second test, at which point the read test starts, which also lasts for 8 seconds.

After the write and read cycle is complete, the test repeats, writing for 8 seconds and then reading for 8 seconds. The test continues until you click the Start button again to stop it.

The Results

The results are where the Blackmagic Disk Speed Test needs the most work. While the Will It Work and How Fast panels provide key information that video professionals need, the two speedometers that measure performance in Mb/s only show the current instantaneous speed.

If you watch the speedometers during a test, they jump around quite a bit. The speed displayed when you click the Start button is only the speed at that one moment in time. You get no report of average speed or peak speed. Even with this limitation, you get a reasonable ballpark figure for how fast the drive is performing.

Final Thoughts

Blackmagic Disk Speed Test gives a quick test to determine how well a drive performs. It's also useful for measuring how external enclosures perform with the same drive installed in them. Disk Speed Test works well to quickly see how well a storage system performs.

Even without the ability to log peak and average performance during a test, the Blackmagic Disk Speed Test should be part of every Mac enthusiast's suite of benchmarking tools.