It's a 500GB Western Digital MyBook and has all of my data on it, including all of my creative work, 80GB of footage for a movie I'm making, etc. etc., enough for this to be, uhh, pretty bad. A corner of the enclosure has popped out and will not snap back in with gentle pushing.

What is the best option here? Run diagnostics on the drive? (I'm running an iMac with Leopard.) Put everything I can on my computer? Help!

If me, I would open the enclosure and remove the hard drive within. Then install that drive in a new enclosure, or another computer and run the diagnosis.