OK, I've now managed to put a grand total of 4 Amiga floppy drives out of action with bad disks. I should have really learned my lesson by now, but no, I'm an idiot and I apologise for the time I'm wasting here.
I know that the floppy drives are being killed by physically defective disks (I think this time I killed it trying to check it for not being physically defective), and the result is a horrific scraping noise, followed by the disk spouting read errors, then "not a valid DOS disk" errors, then not reading at all.
Does anyone else know anything more about this? Also, feel free to yell at me or whatever for being probably the most negligent Amiga owner in the universe. =(