For all those having the “Read Only” issue...

This excellent product works precisely as advertised. If, like BBurr55, you think it doesn’t function because you get a message saying your drive is "read only” and you need to pick a drive with Read/Write privileges, you might be forgiven for thinking that’s nonsense because, "I’ve been reading and writing to them both before and after the test.” You might be forgiven for thinking that, but you’d still be wrong. Mac example: Though you can read and write to your external drives, that’s because the computer knows you as Administrator (or “system"). Open the Finder and go to the external drive you want to test. If you have right-click enabled on your mouse, do that or go FIle>Get Info or Command+I . When the Info window opens you’ll see at the bottom that under Sharing & Permissions “system" and “wheel" are set to Read & Write, but “everyone" is set to Read only. This is by default to keep hackers and malware from writing to your drive. It sees the App, Blackmagic Disk Speed Test, as “everyone” To run the test click the lock in the lower right-hand corner of the Info Window and enter your password to unlock. Change “everyone" to Read & Write and then close the window. You can now run the speed test. You should change the settings back when finished for security.

Great App, but judging by the complaints you might want to warn about this in advance. Thanks!

NothingToGain about Blackmagic Disk Speed Test