Hmm, that's what I initially thought, but it happened with one disk straight after cleaning the heads with isopropyl.
I probably should of mentioned that!
Some of the disks, especially the ones that scratch, make a horrendous noise when they are spun in a drive. Sounds like nails down a chalkboard.
Anyway, thanks. I'll try with a noisy disk in another drive and see if it still happens.
Amiga 4000: Apollo Turbo 040, 128MB Fast, 40gb HDD, CD-ROM (needs internet)
C64: 1541U-II, MixSID, WiFi, LumaFix64
ZX Spec, A1200, CD32, MacPlus/SE, A500, BBC Turbo Master (NASA), 2600, Acorn 4000, VIC-20, IBM 5155, GameBoy