You would need a WD Expander to add a drive to a Premiere, unless you want to remove the original drive and manually marry it to another drive. I'm not sure if the needed tools are publicly available to marry an XL4 2TB drive to another drive. I don't know of anyone using MFS or similar with this setup. The maximum hard drive size you can use with a TiVo is 2TB, so 2TB internal + 2TB external is the max.
Notice that weaknees requires you to mail in your TiVo to add the 2TB external, so they must be marrying on the drives. And they do keep your existing settings, etc. But for $299, it seems a bit pricey.
The only other option, as I said above is the WD Expander, because a stock TiVo Premiere will not recognize any other type of drive.
Roamio Pro (3TB)
Premiere Elite (2TB)
Two TiVo Minis
Left DirecTV 4/15/07. Left TWC 8/8/11. Now on Comcast Xfinity.