Was backing up 3 partitions on drive 0 using wbackup, drive d (XP). drive c (win 8.1 pro x86) and drive I (win 8.1 pro x64). I have this setup in the task scheduler so I ran it manually as I had just changed video drivers on all 3 partitions.

Using a cmd box the backup ran for the first drive counting up to 99% and backup successful, then the c drive started to run which again got to 99% but then it said "compacting backup 1%" and all the way to 99%, the final drive just did a normal backup.

Why would the 2nd drive be "compacted" and what is compacting after the backup?