I understand what's going wrong. Sorry for the length but it's worth the read. Solution below the picture.
=== PROBLEM ===
At any given time, I may have anywhere from zero to three devices plugged into various USB ports on my laptop. The computer assigns each of these a drive letter as an identifier, starting with the first available letter (usually D:) and moving along the alphabet. It's a dynamic assignment.
Depending on the order in which I connect my devices, my Toshiba (read: backup) drive may be D, or E, or F.
When I go to view the rules of my Backup Set in Norton, the "Where" changes to match the new drive letter (E: this time instead of F:). That's great. But it doesn't actually back up to this drive letter, and the backup will fail.
If I delve further into the problem (From the Norton start page > Tasks > Check Security History > Select "Backup" from the drop-down menu) I can see it unsuccessfully tried to back up to drive F: instead of the appropriate drive E:. (See picture)
That's why I have to continuously resave my Backup Set.

=== SOLUTION ===
Make the drive assignment for the backup drive static, rather than dynamic. The lower drive letters will be changing a lot, so avoid drive assignment confusion by assigning the backup drive a letter towards the end of the alphabet.
In Windows 7: Find Computer Management under Administrative Tools in Control Panel. Alternatively, search Computer Management in the Start menu.
Select Disk Management from the left menu. Right-click the backup drive, select 'Change drive letter and paths.'
Select the drive letter and click Change. Assign a new drive letter (something like Z:).
The drive should now always be recognized as that letter and will not conflict with drive letters assigned to other devices, making the Backup Set run smoothly.