Aten USB print switcher drops other connections

I recently purchased an Aten US 221 peripheral switch in Australia.
The device works very well in that I can switch to print to my Canon MP 610 printer between 2 close proximity Windows XP machines. Trouble is each time it switches all the other USB devices connected at the time (portable hard drives, flash drives etc) get locked out and disappear from view in windows explorer. They only came back when I fully unplug the Aten switch from the printer, unplug all usb cables from switch to computer and reboot - last time I also had to re-install my printer drivers.
Aten sent me to a Windows Hotfix for the Usbser.sys drive - but I'm not sure this is the right thing.
Has anyone got any idea what’s happening here – this happens on both XP machines connected to print..

Blooloo