‘Purge Deleted Files’ will delete all the files from your WD external hard drive that have been deleted from the source HD (your computer in most cases). Usually the WD external HD will back up every file you ever save on your computer, and even if you delete a file on your computer, it will remain on the WD external hard drive. If you choose to ‘Purge Deleted Files’ all files that you have deleted from your computer HD that were backed up on your WD external HD, will now be deleted. Think of it as syncing the hard drive of your computer with the WD external HD, only everything that is on your computer will now will remain on your WD external HD. This option should only really be turned on every now and then, because you risk losing important files if you delete an important from your computer accidently. There will be NO back up on the external HD, use it only as a way to tidy up the external HD every so often. After all the whole idea of a external HD is to free up space on your computer as well as have the important files you need incase something happens to your computer. Hope this helps.