This may be a complete red herring but is this an NVIDIA chipset motherboard. I have trouble with one such motherboard and USB - anything that requires lots of USB bandwidth fails in some way or another. There are cases mentioned on Google and I assume this a bug or quirk in the hardware that the kernel does not (yet) work around adequately.
Steve,
It's an ASUS P5N-E SLI, which I think is designed around the Nvidia nForce 650i SLI chipset.
If this is something you've come across, is there anyway to diagnose further to be more confident that's the issue.
It seems odd that the PC sticks work fine, but these no longer do.
Peter.