a500 floppy drive of doom!

[Zac67]

There may be just a larger piece of dirt on one of the heads, scratching the disks. More probably one of the heads is torn off its suspension, esp. when the scratches appear on the upper side of the disk.
You can use a flashlight to look into the drive; you should be able to see either dirt or torn head. In any case the drive needs to be taken apart to see whether the heads are still OK.

[Zac67]

If the drive still reads the heads can't be loose, so all is not lost.  :-) Those drive cleaning disks haven't worked well for me in the past though (it's 'wet' cleaning I hope, dry cleaning is for the birds).

Cleaning the heads is rather easy: remove the 2-4 screws that hold the top of the drive and gently remove it. You can see the heads; the upper one can be lifted some. Just use a cotton swab with isopropylic alcohol to carefully remove any dirt you can see. I've even used a finger nail to remove bad dirt, just don't use metal tools or you'll easily scratch the head.

Maybe there are even scratches on the head, caused by badly beaten up disks, loose shutter parts & springs - then you're probably out of luck.