3tb internal, 1tb external is the "official" maximums.
There is a thread or two on here about the efforts towards internal 4gb....
Weaknees.com is selling up to 8tb upgrades, for a premium.
$350 for internal 4tb, do it yourself install
$250 for 2tb external, $400 for 4tb
$699 for 8tb, do it yourself install of 4tb internal +4tb external.
-Roamio Pro/Slide Pro
-Stream (Gotta go unplug the sucker one of these days!)