At $DAYJOB, we are having issues with a Mac Mini that is acting up. It crashed on boot, and re-installing macOS didn’t help as it complained about the file system being damaged, no matter if I reformat (“erased” in Apple-speak) or repartition the disk. The built-in Apple Diagnostics tool crashed after about 16 minutes, so I thought I’d run memtest86+ on the machine. But without a working OS boot, I was unable to get it up and running, and googling for information didn’t help.
To get it running, I had to create a bootable USB stick, for which I had to find a Windows machine and run their USB Key installer. However, the disk did not show up in the list of boot options when booting the Mac Mini pressing the Option key. To find it, I had to install rEFInd on a second USB stick (they have a USB flash image ready for download, so no Windows machine needed).
With both USB sticks in the Mac, booting with the Option key let me select the rEFInd USB stick, which in turn found the memtest86+ stick as a “Legacy Windows” image. Now the test started fine.