This is a really frustrating problem. I have two portable external hard drives that have both ceased to work ever since I upgraded to 10.7. These are both used for large video files and I need them to work as they have some important stuff on them.
One is a 1TB WD MyPassport, USB connection.
The other is a 500GB Seagate FreeAgent, FireWire 800 connection.
The drives are both giving me the same weird phenomenon:
I plug them in, and they'll boot up--sometimes right away, sometimes after several minutes--but then they make my Finder worthless. The files won't preview in Cover Flow, and expanding a folder takes forever. When I try opening a video file in QuickTime, I end up with the beach ball of death. I try restarting the computer, but I can't even do that because "Finder (Not Responding)."
So I tried manually shutting down by holding down the power button. When I restarted the computer with the drive attached, the Finder wouldn't launch. The dock would appear, but no icons on my desktop whatsoever. When I unplug the drive, once again hold down the power button to shut down, and restart without the drive attached, everything boots up just fine.