The somewhat-still-working stick, when inserted into the iMac, brings up a pop-up window that says "The disk you inserted was not readable by this computer. Ignore/Eject". Disk Utility sees it as an "Unknown" disk2s2 426KB external drive. But it lets me try to Erase it, although that results in an error saying "Couldn't modify partition map." I tried all four formatting options (Mac OS Extended Journaled, Mac OS Extended Case-sensitive Journaled, ExFAT, MS-DOS (FAT)). I tried tinkering with the Security Options.If it had been one stick, I would just assume the stick had gone bad. But two sticks, at the same time, happening by the same process, sure seems suspicious.
Just to round out the OSes, I'll go see if I can find a Windows machine.