I have three external harddrives to backup my photography work but it's a pain in the ass trying to manually move files around to keep all three drives current. What I'm looking for is a programme that will automatically apply changes to the main harddrive to the two other harddrives. Basically, I want all three drives to be clones of one another. So if I add/delete a file from the main harddrive, that same file will be added/deleted to/from the two other harddrives. Any advice?