Norton Ghost, External archive Drive Letter changes

(Op systems: Windows XP sp3 and Window 7)

I set Norton Ghost to create Recovery Points and backups to my 2Tb external USB drive (a MyBook external drive).

When I configured the backup, the external drive letter was Drive G:

After I returned from a trip and replugged the external drive to my laptop, the external USB drive connects as Drive E:

Now the backup doesn't work.

How do I configure the backup to go to the external drive regardless of the drive letter which Windows assigns to the drive?