Tried and True method to shake out a USB problem?

Well...I wish I knew...>Bought an identical unit. I set both up on 2 lappy and ran em through and through at the same time for 4 gigs. No problems. All I did is unplug power supply from lappy then attached USB then reconnected power supply to lappy's. No problem. I hate this because I can't replicate what's going on during live play. No problems so far with this method. hmmm