Affliction

Anyone else having problems with copying to USB devices on Win7? Every time I try to copy a larg-ish file (e.g. a film or tv show) to a USB stick, ext. HDD etc., it copies the first half increidably fast, then disconnects the device, and every other USB device, apart from the keyboard and mouse. The only way to revive it after this happens is to re-start the PC. Anyone got any suggestions? It's doing my head in