Back up overwrites existing data

MyBook had folders with pictures on the drive until a backup was run.  Now the backup is the only thing on the drive. The backup has only the files from the computer and none of the pictures originally on the external drive.  This happened in the past and i used data recovery software but still can’t find the original files that should have been separate from the backup.  I was working on another friends computer last night and it seems the same thing happened.  A while back I copied all the pictures from the compter and other external drives to the MyBook 640 and when i looked I found that the only files on the drive are the backup files. Does a backup overwrite existing files and how can they be invisable to data recovery software?


The backup should not delete or overwrite any files on the drive.

The backup engine sole purpose is to save data on the drive not delete.

There are certain applications that need the drive to be erased in order for them to work. For example, if you used a Windows formatted drive and try to use Time Machine the software will request the drive to be formatted before it can start the backup.

What backup software are you using?