To clone your existing HDD to a new one, the software we use here for Tivos is Winmfs Beta 9.3G, or a Linux bootable JMFS CD.  That way you keep all your recordings, and it can expand partitions to fill a 1TB disk. If you PM me, I can give you a link in my Dropbox. As Greg says, go with a WD Purple Surveillance  drive as the best option. 1TB is the easiest option. Your existing disk, if it's then original, will be a 320GB WD green manufactured in 2009 so it'll be on borrowed time. There were also a small number of 160GB drives in the initial trial shipment.

Note: many people blame their HDD for being noisy when it's actually the fan mounted inside the back panel. Most of these go noisy after a few years and need cleaning and lubing.