I am beginning to think the only way I am going to get a working Macintosh SE is to buy a working Macintosh SE.

I tried a different floppy cable.  That made no difference.

I tried installing a different floppy drive.  That made no difference.

It could be that the floppy drive in the machine and the floppy drive I installed are both bad, as neither have been tested in anther machine.

Or it could be that something is wrong with the SE motherboard such that it won't recognize any floppy disks.

I have no idea.

I do know that the floppy disk works, as a Macintosh Plus boots from it just fine.  (Both before and after testing on the SE, just to make sure the SE didn't ruin the disk.)

And, right now, the original floppy drive in the SE won't eject disks, so I've messed something up with it somewhere.  Or, rather, I made it worse because while it wouldn't boot from the floppy disk before, it would at least eject it.