i've seen others asking about this, found the solution and thought i'd post i hopes that it helps others.
the link provided below explains the problem (changing or reformatting external usb hard drive used for backup yet vista still thinks there are backups on that drive) and gives the solution at the end of the thread.
sushil baid [MSFT] wrote the solution
http://www.jlaforums.com/viewtopic.php?t=3590580&view=next
note that you will have to use an elevated rights command prompt (right-click on command prompt icon and choose run as admin)
use
wbadmin delete catalog
to delete the history, but before doing so use
wbadmin get versions
to see what backup dates are stored in the catalog
use
wbadmin /?
to see what other commands you can use
good luck!
john
