I am also in the same boat. I'm new to all things Apple, and I can't figure out why I'm getting the same error message. I bought an Airport Extreme base station and plugged in my USB 2TB WD external hard drive thinking I could get a decent NAS. After I set up the NAS, I can connect for a while. However, the next day when I try to look or put something on the NAS, I cannot see or access it. When I physically look at the drive, it's still powered up.
Any ideas for an OS X rookie?