Just went through a similar issue-- I am assuming that both your drives are attached through the USB port. Check this thread-- it may help. http://social.technet.microsoft.com/Forums/en-US/w7itprohardware/thread/3aae3b66-6a1a-47e8-ad1b-b20b68eaecf8#79ea3219-d76e-40bc-b910-c7d347002e66
It only happens when I try to copy files to and from my USB external drives. I have two-- one is a Western Digital MyBook 160 GB drive and the other is an Iomega 1 TB drive. Same result with either one.