Make sure to select a place in the directory structure you can write to...

For all those having “Read-only” problems, all you should need to do is to navigate down to a point in the directory structure where you know that you have full read-write privileges. So, if you want to try testing the OS boot drive, navigate down to your home directory, and you should be done. If there are other volumes you want to test, you need to make sure you do so in a place in the respective directory structure where you have full read/write privileges.

That said, this tool only seems to test sequential I/O, not random I/O. And it gave me some wildly varying numbers (like, by a factor of two) on my internal SSD, as well as other devices I tried it on. Finally, I also have iStat Menus and LittleSnitch Network Monitor installed, and while BlackMagic was doing the disk testing, I didn’t see a single blip in the disk I/O or network I/O meters, which tells me that something really strange is going on.

I’m going to try some other disk benchmarking tools that will hopefully include random I/O as well as sequential I/O, and see how that works out.

BradKnowles about Blackmagic Disk Speed Test, v2.2.2