I have 2 external USB drives connected to my iMac running 10.7.3. One is designated completely for Time Machine use and the other I use for data. (Nothing but a bunch of JPGs on that one.) The first spins up once an hour for Time Machine backups, great. But both of them spin up at random times while I'm doing, well, ordinary stuff, nothing disk-related. Maybe just switching to another app in the dock, or surfing to a new web page.
Spin-ups for no clear reason are a bit annoying, but I can accept them as long as I can continue working. But in some cases, the spinning pizza cursor appears, and I'm locked out from doing anything while the cycle completes. That does not make sense to me. It's a multi (4) processor, fully multi-tasking system. All I/O should be asynchronous. Finder/GUI operations should not be affected.
Any ideas why these random spin-ups occur? How to search or instrument MacOS to find out why? How to limit or stop them?
Disconnecting or un-mounting the external disks will work, but is not an acceptable fix.