Bought a new 2TB external USB drive by Samsung for the iMac. I know Samsung doesn’t like Apple but this is ridiculous. Whenever the computer goes to sleep the drive ejects, and when the computer wakes up again, the drive doesn’t.
Looking into this I discovered this has been a common problem for a lot of people. It seems several brands/models exhibit this behavior. It’s not limited to Lion btw, older versions are affected as well. So buyer beware.
In my case the problem was easily ‘fixed’ by swapping it with one of my Mac Mini’s drives as that one runs 24/7 and never sleeps (being used as a server and all).