externalization

i have a harddrive that is split up internally, where one portion i keep as an exact backup of my computer, and the other is for extra storage. today i wanted to back up my computer, and so erased the old backup data, and then used copy program to move my entire computer over to the drive. There's an option when you copy, which is to delete anything that's on the backup drive if it isn't also on the original drive. If you uncheck it, the program won't delete anything, will just copy stuff over. For some reason I checked it, despite having just erased everything already. Then for some reason I told it copy to the half where I keep the extra storage. I realized this mistake about 1.5 hours into the copy, which was also 10 seconds before a message popped up saying "Copy Successful." I now have a drive that is my exact computer on one half and absolutely nothing on the other.