Hi Brovig,
1. How many drives are there in the computer?
2. Can you see the external hard drive in Disk management?
To open Disk management follow the steps below.
1. Open Computer Management by clicking theStart button, clicking Control Panel, clicking System and Security, clicking Administrative Tools, and then double-clickingComputer Management. If you're prompted for an administrator password or confirmation, type the password or provide confirmation.
2. In the left pane, underStorage, click Disk Management.
A system image is an exact copy of a drive. By default, a system image includes the drives required for Windows to run. If you select an another drive to backup, then you may get the option of D drive to backup.
You can only save your backups on a network location on Windows 7 Professional, Windows 7 Ultimate, and Windows 7 Enterprise.
You may refer the below mentioned links for more information.
http://www.microsoft.com/windows/windows-7/features/backup-and-restore.aspx
http://windows.microsoft.com/en-us/windows7/products/features/backup-and-restore
http://windows.microsoft.com/en-us/windows7/Back-up-your-programs-system-settings-and-files
Thanks and regards,
Fouzan – Microsoft support.
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