Have you tried using another usb lead? Could it be that there is something dodgy about the connectors which when the plug is inserted with power on screws everything up, but once the plug is seated, will work ok, at least for the time being?

Presumably it does not happen with other external drives, which would rule out some malfunction with the socket. Or one could try with different sockets to make absolutely sure.

I can't think of any other explanation. It can't be the drive itself or it would happen regardless of when power comes on. If it happens with other leads, I would return it (though I can't think of how the drive itself could be defective in a way to cause this).

I would also try it first with another computer and see if it does it to every one. Have you done that?

Very strange.

Al