Have you tried turning it off and then back on?
Usually when you get an error message like that, it means that the device dropped off the USB bus without any warning to the system.
Usually it means that the drive just died or the USB interface in the external case, failed.
That's probably not the case if the other drive is seen and is functioning.
Ahh, the beauty of RAID.
I am running OSX On aniMac, I have 2 Lacie 2Tb external Hard Drives connected. The system only recognizes one of them , it is a big disk extreme+. This is after regular use for over a year, recently it started acting up a little noisier than usual. But it was fully operational. About 3 days ago I got an error message warning me that I need to eject the drive properly and after that the drive was not seen on the desktop. I can not even run Diskwarrior to diagnose the problem. Any suggestions?