If this continues (and I have had the same issues on occasions) you most certainly have a hardware problem–most likely the cable or the control card in the drive. It will drive you absolutely batty if it is the control card because you will see the drive sometimes, and at others no—Rebooting will often get it back, but at other times no………..THats why I know longer use mainstream external enclosures—only premium for me-and better yet, I try to avoid external drives altogether.