You need to format the disk '?' before you can use it

So i've searched the forums, and have came to similar problems. The problem is as follows. Someone wanted me to back up their data from two computers, both with windows xp onto an external hdd. I plugged in a brand new external and both computers were able to read/ write to them without any issues. Once I backed up all the data, I then proceeded to install windows 7 on a laptop. I...