Watch out, there are 3 different backup things in OA and they need to each be in their own directories.

  - Value archives, long term trending storage (each one setup independently)
  - alert archive, if you initialize the folder, it will delete everything (all folders under that will be erased)
  - the snapshot backup of the entire project.

Keep each of those in different locations. Can be in different folders on same drive.

I recommend that you don't backup to the same physical disk as the running project.  Think about a NAS, SAN, or even a USB is better than the system disk!