External USB hard drive disconnects

I have a Maxtor Diamond Max 500g SATA drive in an external Thermaltake enclosure that uses USB. For some unknown reason, it keeps disconnecting from my iMac (Intel running OSX 10.5.5). I want to leave the system running and just be able to have it be there when I return, but it keeps disappearing from the system and has to be re-mounted.

Can anybody give me a hint on what to try? I've googled for solutions, but nothing I've found seems to be what I need. (And nothing has worked.)

TIA

Never mind--duff power supply was driving the motors nuts. Finally died. New one on order.