When trying to performa a backup to an external USB drive you may get the following error:

The backup was not successful. The error is: There is not enough free space on the backup storage location to back up the data. (0x80780048)

When trying to perform a backup across peer to peer network from Windows 7 computer to the Windows XP computer you might get the following error:

Error code 0x81000039, remote share path not available

You should change the NTLM authentication level on Windows 7 machine:

Hit Start. Enter “gpedit.msc” in the Start Search box.
Navigate to: Computer Configuration | Windows Settings | Security Settings | Local Policies | Security Options
In the right pane, double-click on “Network security: LAN Manager authentication level” setting.
Change the level to “Send LM & NTLM - use NTLMv2 session security if negotiated”.

You may experience the following error if using Windows Backup on a Windows 7 machine:

Cannot write to the drive, Error Code 0x80780047

Solution:
Backup and restore will create seperate folders on the external drive for the machines being backed up. However, the system image is placed outside the folders and the rest of the backup is placed inside the folder. Backup and restore only allows for 1 system image to be stored on a drive.

When you try to backup a second machine, the backup starts  to write to the correct folder. When Backup trys to save the system image, it finds that it already exists and the error mentioned above is presented. 
The best option is to partition the drive for each of the PC's, so backup job can excecute normally.

One of Windows Backup errors that you may experience while using Windows 7 is:

The backup was not successful. The error is: Windows Backup encountered a problem while determining additional locations of one of the users included in backup. (0x81000038)