It's interesting that you're having problems with the auto-inject drives. I used to find them quite reliable. I couldn't say the same for the manual-inject ones though!

One thing I have found that is "weird" about Mac floppy drives is that when you format a disk, the Mac always seems to take a look at the disk first. Often the Macs don't like what they see and then fail to format a perfectly good disk in a perfectly good drive - in fact they don't even try to format it, but eject it with an error message.

The fix is to erase the disk with a degaussing wand (used for fixing CRT monitors) or a bulk tape eraser. The same disk that no Mac can format will then format perfectly and go on to be used reliably for a long time to come. I "repaired" countless "faulty" floppy disks using this technique back when floppy disks were all the rage (and the disks were quite expensive).

... and if the head gimbal (that copper spring thing the head sits in) is bent or damaged, I don't like your chances of repairing it!