Weird...
I don't know why it is showing up as the D drive, but go to Disk Management.
right-click on My Computer, Manage, Disk Management
top window, right-click on the external drive, Change Drive Letter and Paths, change it to F: or M: or something. Ignore the warning and do it.
If that doesn't fix it, I don't know, maybe try it on another computer. Or you can just erase it and start over. From Disk Management, lower window, find the drive and you'll see the partition with a colored bar. Right-click on the colored bar and do Delete Partition. Once it's done and Unallocated, right-click again and do Create Partition. Do a quick format.
Once that's done, try the backup again.
