When logged in as admin you can install USBDLM :
http://www.uwe-sieber.de/usbdlm_e.htmlUse the inf file to always make it assign usb drive(s) the same letters and see if there is some way of using group policy to give them rights or if you have the usb drive inserted whilst logged in as admin see if there are any security tab(s) on the drive when you go to the properties of the drive ( If so add those users and give them full rights to the drive(s)
Just a thought