File Backup Space

I am using the Vista File Backup program that comes with Vista Ultimate.
I am backing up to a 500GB USB disk. The initial backup takes up
approximately 125GB. I have the automatic backup set to 1AM each day.
I use the computer
in business with some Word and Excel documents being added or changed
daily.

The backup functions properly but fills the 500GB USB disk up in
approximately 10 days, at which point I see an error message that the
backup
failed due to lack of space. I have attempted deleting earlier backups
and selecting "try again", but then the error message reads "cannot find
backup "mm/dd/yyyy" (which is the latest partial or failed backup). The
only way I have discovered to make the backup program work is to delete
the existing backup and start a new backup that will again last for
approximately 10 more days.

Does the backup program make incremental backups after the initial file
backup or does it do a complete file backup periodically? Should I be
able to delete or groom older incremental backups to make space for
newer incremental backups without deleting the complete backup and
starting over again with a complete new File Backup?

Regards,
Gary


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allgevista64