"MDT" hard drives are used drives that have been refurbished by a company called Magnetic Data Technologies - they do not manufacturer drives themselves. The drives are manufactured by WD (Western Digital) or Seagate, but when MDT refurbishes them they remove all the WD or Seagate labels.
I bought an MDT refurbished hard drive once but I will never buy another one - it failed after only four weeks.
They are cheaper than a new hard drive but that's because they have already been used. Buy a drive that is really new.