I've seen a few threads on this issue, but haven't found an answer. I have a 750gb external USB LaCie drive as Time Machine backup, but when the Imac I7 sleeps, the drive will behave as if it has been disconnected. There's the "you should eject disk before..." message with warnings about damage. I suppose I could disable sleep mode, but does anyone know what's going on? My external firewire drive never has this problem.