If you are doing a backup, what kind? Are you using the Win 7 file backup?
Do the drives you are trying to access already have a file backup on them and enough room for the backup?
Are the drives encrypted or otherwise not available for your use?
Windows may not want to access a location for different reasons. Possibly you are trying to overwrite something it won't allow. Maybe the process cannot access a specific file location or possibly it doesn't have the privileges to access the area. If you were trying to write to a location which is part of the basic Windows install, it may not allow that.