yeah you need a RAID 1 drive array. You can have a hardware or software RAID controller but i'd prefer to go the hardwired route; less to go wrong.
That means 2 hard drives, and doubles your costs. The cheapest way to do this is to install a new hard drive and sort out a RAID controller for your PC (only needs one new drive). However, if oyu want an off-the-shelf product, i'd recommend one of these:
Freecom