I had some similar intermittent behavior using external USB box (see if you can find thread, I don't recall all the details right now, I'll try to find later for pointer). I never did resolve the issue, but was eventually able to get a complete backup/restore after several tries on each... Mine would not see the external disk after the reset when trying to restore more often than not. I was using (acomdata?) combo SATA/PATA converter and guessed it was getting a timeout (the combo converter takes longer to respond/recover from reset?). I have no solid data though since it was so intermittent.