Joe,
I replaced the HD in my TIV, it's pretty easy to do.
Go to
this site and download the boot cd (MFS TOOLS 2) and follow the instructions for your particular setup/desire.
Rather than spend $20 on a $2 piece of plastic, I just pulled my old hard drive and replaced it with a 160gb Seagate drive on which I copied ALL my old tv shows that I had saved. It took me about 30 minutes of actual work to get everything setup and about 2 or so hours to transfer all my shows from one drive to the next.
Actually, now that I think about it, I didn't mount the second drive because I didn't want to wait to get the HD mounting bracket.