External Hard Drive Constantly Active

I have a Western Digital MyBook Studio attached to my iMac with two partitions. One partition for Time Machine and the other for my media (music and movies) which is linked to iTunes.
For the last couple days, the drive has been active a lot which has caused it to get hot and make the fan run. I have closed all my applications and the drive continues to be active. How can I find out what is reading/writing to the drive so I can make it stop?

You probably need to disable drive indexing:
"DriveLabel" should be whatever it shows up as on the desktop. If you're unsure, you can press the tab key after you type "/Volumes/" to list all possibilities. You might also consider "ejecting" the drive and turning it off when you don't need it.