I was also confused by the statement about the mains power. It's what is being delivered by the switch-mode power supply inside the computer that matters. Disk Drives don't run on AC

That's true and I think I might have just had a run of bad luck. The last one that failed 3 days ago was one I put in a Inateck docking station which is powered off the mains and data connected via USB. Went into Windows DiskManagement and it would not even show up as a drive. Did not loose anything as I keep multiple backups. I could try the old freezer trick from days gone where they say the cold temp can sometimes bring a flash drive back. Never seen it happen and probably not worth the time to do it unless I get bored.