Mac Disk Speed Test: How to Run One
To run a disk speed test on a Mac, you use a benchmarking app that measures read and write performance—the most common choice is Blackmagic Disk Speed Test, available free on the Mac App Store. You open the app, select the drive or volume you want to test, and click Start; it shows real-time MB/s for writes and reads, plus a verdict on whether the drive can handle common video formats.
What a Disk Speed Test Actually Measures
A disk speed test reports two numbers: sequential write speed and sequential read speed, usually in megabytes per second (MB/s). Sequential means the drive is reading or writing one long, continuous stream of data—like copying a large video file. This is different from random I/O (small, scattered reads and writes), which matters more for database work or launching many small apps, but most consumer benchmarks focus on sequential because it’s easier to compare and directly relevant to media workflows.
The test writes a temporary file (often 1–5 GB) to the drive, measures how long it takes, then reads it back. The result is a raw throughput number. Some tools also report IOPS (input/output operations per second) for random access, but that’s more common in enterprise storage testing.
Why You Might Need to Run One
People typically run a disk speed test to:
- Verify a new SSD is performing as advertised (e.g., a NVMe drive should hit 3,000+ MB/s, while a SATA SSD tops out around 550 MB/s).
- Diagnose slow performance—if your Mac feels sluggish, a drive that’s far below spec could be failing or throttling.
- Check if an external drive is fast enough for video editing, large file transfers, or running a virtual machine.
- Compare two drives before buying or choosing which one to use as a boot volume.
It’s also useful after updating macOS or replacing a drive, to confirm everything is working.
How to Run a Disk Speed Test on a Mac
Here’s the general process, using the most popular free tool:
- Download Blackmagic Disk Speed Test from the Mac App Store (it’s free).
- Open the app. You’ll see a window with a large Start button and a settings gear icon.
- Click the gear icon to choose the target disk. By default it tests your startup disk. You can select an external drive or a specific volume.
- Click Start. The test runs for about 5–10 seconds per pass; it will loop until you stop it.
- Watch the two large numbers: the top one is write speed, the bottom is read speed (both in MB/s).
- Stop the test after a few passes. The results are shown in the main window, and you can also click the “Show Results� button to see a history.
If you want a more technical test, AmorphousDiskMark (free, from the Mac App Store) offers sequential and random tests with adjustable block sizes and queue depths.
Interpreting the Results
What’s “good� depends on the drive type and connection:
- Internal SSD (NVMe): 2,000–7,000 MB/s. Apple’s newer Macs with PCIe 4.0/5.0 can exceed 5,000 MB/s.
- Internal SSD (SATA): ~500 MB/s (limited by the SATA III interface).
- External SSD (Thunderbolt 3/4): 1,000–3,000 MB/s, depending on the drive.
- External SSD (USB 3.1/3.2): 500–1,000 MB/s.
- External HDD (USB 3.0): 100–200 MB/s.
- Internal HDD (5400/7200 rpm): 80–160 MB/s.
If your numbers are drastically lower than expected, check the connection (cable, port, enclosure), whether the drive is nearly full (SSDs slow down when they’re >90% full), and if TRIM is enabled for SSDs (it is by default on internal drives, but not always on external ones).
Common Mistakes and Misconceptions
A frequent mistake is testing a drive that’s almost full. SSDs have less spare area for wear leveling, which can cut write speeds significantly. Another is using a USB hub or a poor-quality cable—this can bottleneck the connection and make a fast drive look slow.
People also confuse MB/s with Mbps (megabits per second). A result of 500 MB/s is 4,000 Mbps—a huge difference. Always check the units.
One misconception is that a single test tells you everything. Real-world performance varies with file size, queue depth, and the drive’s cache behavior. Some drives have a fast SLC cache that fills up during large writes, causing speed to drop after a few GB. Run the test for longer (or use a tool that lets you set a larger test file) to see sustained performance.
Limitations of Disk Speed Tests
These benchmarks measure raw throughput, not overall system responsiveness. A drive with great sequential speeds can still feel slow if its random I/O is poor or if the controller has high latency. Also, the test file is written to the free space on the drive, so results can vary based on how much space is available and where the file lands.
For external drives, the test measures the drive plus the enclosure and cable. If you get low numbers, try a different cable or port before blaming the drive itself.
Finally, the test doesn’t tell you about the drive’s health (like pending bad sectors or wear). For that, use Disk Utility’s First Aid or a tool like smartctl (via Homebrew) to check S.M.A.R.T. status.
Alternatives to Blackmagic Disk Speed Test
Several other free and paid tools exist:
- AmorphousDiskMark: More detailed, with random read/write tests and adjustable parameters.
- QuickBench: A lightweight tool that tests sequential and random speeds, plus file duplication.
- DiskTester: Allows custom test sizes and runs multiple passes.
- iostat (Terminal): Built into macOS, but it shows live disk activity, not a synthetic benchmark.
- dd (Terminal): You can manually time a write and read with the
ddcommand, but it’s risky—if you mistype the output path, you could overwrite data.
For most users, Blackmagic Disk Speed Test is sufficient. It’s free, simple, and widely used in the film industry as a standard for checking if a drive can handle certain video formats.
Frequently Asked Questions
Is it safe to run a disk speed test on my Mac?
Yes, it’s safe. The test writes a temporary file and then deletes it. It doesn’t touch your personal files. However, it does cause extra write wear on SSDs, but the amount is negligible—a few GB per run, and modern SSDs can handle hundreds of TB of writes.
Why is my external drive slower than advertised?
Advertised speeds are often theoretical maximums. Real-world speeds depend on the connection (USB 3.0 vs. Thunderbolt), the drive’s cache, the file system, and the computer’s port. Also, many external drives use SATA SSDs, which cap around 550 MB/s, even if the enclosure supports faster.
Can I test the speed of an SD card or USB flash drive?
Yes, the same tools work. Just select the card or drive in the app. Expect much lower numbers—SD cards typically range from 20–200 MB/s, and USB sticks are often 10–150 MB/s. This is useful to verify a card is genuine and not a fake with inflated capacity.
What is a good read/write speed for video editing?
It depends on the video codec and resolution. For 4K ProRes 422, you need about 200 MB/s. For 8K raw, you may need 1,000+ MB/s. Blackmagic Disk Speed Test actually shows a list of video formats and whether the drive can handle them—that’s a practical guide.
Does a full drive affect speed test results?
Yes, especially on SSDs. When a drive is nearly full, the controller has fewer free blocks to write to, so write speeds can drop. For accurate results, ensure at least 10–20% free space.
How do I check disk health without a speed test?
Open Disk Utility, select the drive, and click First Aid. It checks the file system and some hardware errors. For more detail, use Terminal with smartctl (from smartmontools) to view S.M.A.R.T. attributes like reallocated sectors and wear leveling.
Related
Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.