You cannot upgrade the internal without removing the external first, and it would also result in losing your recordings. You could transfer them to computer or another Tivo before removing.
To remove external, you must shut off Tivo and External, unplug the cable to external, plug back in Tivo and follow on-screen to confirm divorcing drive.
Original S1 (Recycle Heaven)
Series 2 (PLS, sold)
TiVo HD (XL, PLS, "652")
Juliet Basic (4XL, PLS)
The TiVo Apocalypse begins on Sat 2013 Feb 16 00:00 GMT!