The Premiere won't accept the original 320gb drive as an external anyway. The drive needs to come from a WD DVR Expander for the TiVo to use it on a Premiere as an external. And as Steve said, once you upgrade to the 2TB, I don't think you can add an external.
Save the 320 as a backup. I just used mine today to resurrect a Premiere that had a failed drive.
Premiere Elite (2TB) on MoCA
Premiere (2TB upgrade) w/ MoCA adapter
Premiere (320GB) on ethernet (was to be a Mini - too late)
Left DirecTV 4/15/07. Left TWC 8/8/11. Forced back to TWC 3/7/13.