what he said

i hear slaved drives and cable selected can be a bit finicky with some systems

I had a system which used to do something similar and I used to leave it occasionally to sort itself out, i think that sometimes the system tries to dump stuff from RAM to HDD swap file and if it gets to picking stuff up and dumping it in a cycle (like neurotic swapping) it could explain some of it. My wild hunch sais incessant disc access on the master might obscure a slave drive or secondary ide line master CDRW, but I am completely guessing here.

mine did it much less after an OS reinstallation

just a thought - RAMpage utility has a RAM defragmenter as well as a dump and clearance capability so it might be worth a try to see if it stops it, there is a link a few posts down.

also did you ctrl alt del to see what was running ?