I'm sure I could try that, but then I'd be left with two dead 800K drives (the ones from my Plus and SE). I currently have no other drives to work with (aside from my external 800K drive).
Since my original questions were on how to diagnose and repair the drives, this is still my goal.
Both drives have been cleaned and lubed per this site's wiki, and all 4 gears in each of the drives are unbroken. The disk that boots fine in the external drive (Symantec emergency disk) does not work in my other drives. In the Plus' drive, I hear a repeated series of three quiet whirring sounds. In the SE's drive, all I hear is a humming noise. The disk had to be manually ejected from both drives.
Is it possible I have installed the eject motor incorrectly? Should I have tried to move the metal cog on the eject motor (the one that drives the smallest gear) to confirm that it hasn't seized?
I'm missing something, I just don't know what it is...
Thanks!