First of all, try another USB port. If that doesn't work, try this, in order. stop after the diamond works:
1- uninstall/reinstall activesync, then try accessing files with it.
2- Stop the Diamond usb drive by clicking the safely remove device icon. Then plug it again.
3- Uninstall all USB Root hub in your windows device manager, then reboot computer.
4- Hard Reset device.
5- Reinstall Windows
6- Return Device.