Linc- I hope you are still here on this thread or can see it, and respond.
I have very similar situation occurring.
First the machine in Question is an iMac 27" running 10.7.4.
About 3 days ago I hooked up an old external 2TB LaCie hard drive to check to see what was on that drive. Immediately, the drive went into a "pulsing" mode and the i Mac began to act weird. I shut it down completely thinking I would try to solve the problem today.
I am trying to get the finder and applications back to normal. The machine will recognize the non-corrupted external hard drives. It sees the mouse and keyboard. It sometimes does see the Apple Menu and the menu bar and at other times does not.
I live in Montana- about 1100 miles from any Apple Store...Portland would be the closest..
1) I'd like to understand how hooking up the LaCie drive "corrupted" the main machine.
2) Is returning to Normal - a real option without extensive repairs?
Here is what goes on:
a) mouse clicks on any item in the Apple menu, including the Apple it self results in nothing.
b) clicking on any items in the application are of the dock, resutls in nothing
c) clicking on any folder on the desktop also results in nothing.
Backup are limited at that was what I was seeking- a drive to hookup to back up everything prior to the beginning of an edit session... HELP! Please