Well on a factory reset the os is put at the start of te disc and possible a small 10G partition is put at the end of the disc to hold the factory reset info..
However if you have data in middle of te disk there is every chance you may recover it fine
try recovermyfiles from get data http://www.getdata.com
or for free that works well but aint too great at adding directory structure there is:http://www.piriform.com/recuva
John J. www.thinkonline.co.uk