I'm having this same exact issue. Inserting a USB flash drive works fine, but once I start copying files over to it, it will stop after about a minute, and then vanish. Device Manager shows an "Unknown Device". Same thing happens when trying to sync my iPhone. These devices worked fine when I was using Vista Ultimate x86.
I was able to get around it by installing an NEC USB PCI expansion card, and not using the builtin USB ports on my motherboard (Biostar GF7050V-M7). This fixes it, but I had to remove my gigabit ethernet card to make room for the USB card. I have a feeling that the default drivers for the USB controllers don't work well with the USB chipset on my motherboard (which appears to be an Nvidia nForce, not 100% certain). I've tried installing nForce drivers straight from Nvidia but this didn't help.
I hope we can keep this thread alive until someone finds a more solid solution.