Unusual File Renaming Behavior?

(Running Vista Ultimate, 64-bit, SP-1 installed. Everything
seems to work just fine.)

As part of my monthly back-up process I have been making
copies of my Contacts file (renaming it BUcontactsdate) and
Favorites file (renaming it BUfavoritesdate). Until I
installed SP-1 the name changes "took" on the back-up files
which were located on either external drives, or DVDs. Now,
I have noticed that most of the time (but not always) the
renamed files revert immediately to their earlier name when
I access them on the external drive or DVD.

This is not a major - or even a minor - problem for me, but
I'm curious about this behavior. Can anyone explain it?
Can anyone suggest an easy way to rename copies of those
files and have the new names remain?

Many thanks.

--
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CWLee
Former slayer of dragons; practice now limited to sacred
cows. Believing we should hire for quality, not quotas, and
promote for performance, not preferences.