Ummmm.... RAID is not a backup policy. Fine use hotswap and rsync or similar but you really shouldn't be relying on raid rebuilds for that.
These are backups to begin with. It is the archive disk for backuppc which contains millions of hardlinked files for the de-duplication it uses. It would
Ouch! I understand now. Backuppc does leave an unholy mess of the filesystem. Using RAID isn't a good solution but I guess is almost the only one that works.