I've got a 2012 with a 125Gb SSD and 1Tb HDD that I made into a Fusion Drive. While the HDD hasn't failed yet, I've noticed that I can hear scratchy noises coming from it so now's probably a good time to give my Mini a proper overhaul. I'll probably replace both drives with a single 1Tb SSD. The OS has accumulated quite a bit of cruft over the years so rather I'd rather do a clean install than restoring from a backup. I've not quite decided what to do with the older SSD yet. If I were to leave it in, would I have to do a a Fusion Drive de-conversion before unplugging the HDD? I assume I don't have to do anything if I'm simply replacing both with a single SSD.