I agree.
The symptoms really does sound like a drive failure and as kwbMitel suggests, if one has fried it's only a matter of time (days, weeks or months) before the other goes. Replace them both. (pay me now else pay me later).
Turning off the nightly activity transfer is not a real good idea long term because of memory fragmentation. At some point it will run out of resources and go down. Depending on the amount of CP activity that could happen in as little as a week or could go several weeks.
On the old SG there was a particular load, G or H-stream I recall, where following the nightly activity transfer the switch would just go to sleep. You'd get called in and everything "looked" normal, CPU activity on the MC, occasional drive activity, no alarms, no logs, but all call processing had ceased and no dial tone. A forced activity transfer would wake it up.