When I attempt a system image back up to a hard drive with 2 TB capacity, I get this error message -" There is not enough disk space to create a volume shadow copy" and then it goes on about volumes, whatever they may be. The "Which drives do you want to include in your back-up?" window tells me the space required is 185 GB, and the space available on the Seagate external hard drive is 1.78TB. What is going on? Any suggestions for alternative back up methods?