Indeed, you can simply buy a ready-made replacement drive from a place like PTVupgrade or Weaknees. In this scenario, you simply replace the drive in your TiVo yourself (they provide all the instructions for you to do it). However, know that this method does NOT preserve anything you have on your current drive. So you have to willing to basically abandon whatever is on your old drive.
If you are capable and have the inclination to do so, and you want to preserve what you already have on your existing drive (recordings, settings, season passes, etc), then something like MFS Live is the way to go.
I agree that you do not want to go to a multi-drive setup. There are issues with power at startup (the TiVo power supply is not sized to bring 2 drives up simultaneously; Weaknees sells a solution to this), and once you "marry" 2 drives together, if one fails its non-trivial to "divorce" the drives. More importantly, for how ever much space you can get with 2 drives, you can get "a lot" of space with just one drive these days. For example, when I fixed my 80hr unit, I replaced the drive with a 320GB drive. Plenty of space for me, for about $100 at the time.
Mark H. Granoff
TiVo Premier XL, Lifetime
TiVo S2 240080, Lifetime, 576 hrs (Maxtor 500GB)
TiVo S2 540040, Monthly, 224 hrs (Maxtor 200GB)
100Mbit Wired Ethernet w/ Netgear USB 2.0 FA-120 Adapters