I have three pc's and would like to keep a system back up of all of them on an external HD. I have tried to do two but when I backed up the second then the first pc's back up vanished or deleted, I put the three back up files in a folder and titled it and was going to do the same with the second, it created the files on the drive but the previous folder was empty. Can only one pc be backed up on a drive or does the back up only show up for the specific pc when its connected,is there something im not doing right.