how do i replace the hard drive - DirecTV DVR

For how do i replace the hard drive, relating to DirecTV DVRs and receivers. My RCA Direct TV dvr has begun stopping and slowing down. I've done everything they tell me (reboot) but still very slow. I am trying to replace the hard drive. It has a Western Digital 80 gig there now. I bought a new Western Digital 160 gig and put it in but DirectTV never went past the powering up stage. Is there anyting esle I have to do to replace the hard drive in my Direct TV RCA DVR? Thanks