I just did the same thing for a set of 40 computers and then another
150. As someone so kindly pointed out to me: Clonezilla. Just put the
disk into the drive, start the computer, make sure to have your new hard
drive and your old hard drive attached some how, and then follow the
onscreen instructions.

I used a external drive that I can swap out hard drives on. It takes
about 4minutes to do 50gb. Though it might take that long for smaller or
slightly larger drives too.