Mac Pro randomly disconnecting USB devices

Hello All,

Am hoping someone out there will be able to advise me. I have a 1TB external SATA drive connected directly to one of my Mac Pro USB ports, (that is, not via a USB hub). The Mac Pro has a habit of randomly disconnecting the drive during file transfers. It seems to particularly do it when the screen saver initiates itself. I've disabled the screen saver but unfortunately that hasn't resolved the problem. The problem itself doesn't seem to be isolated to the external USB hard drive either. The Mac has on occasions disconnected other USB devices too.

Any suggestions or ideas on what the cause of it is and how I resolve it?

Many thanks

GPJ007