Win 7 - Changing Extl HD for Backup

I have 2 computers (a primary and a secondary computer) running Win 7.
I have used Win 7 "Backup and Restore" to make a "Backup Set" and a
"WindowsImageBackup" for the primary computer on a small External HD
drive attached to it. However, today I got a large external HD and
would like to attach it to the primary computer and give the smaller
external HD to the secondary computer, for it to do backups onto (among
other things). I have 3 questions. I have full administrator privileges.

1. How do I erase the "WindowsImageBackup" for my primary computer from
the small external HD?
2. How do I erase the "Backup set" for my primary computer from the
small external HD?
3. Will there be anything special I have to do to make another
"WindowsImageBackup" onto the new larger external HD?

Thank you in advance for your help.
BobS