Well, it depends. DirectTV does not provide a HD21-700. So it's a little hard to be precise.
Now, the HR21 IS a DirecTV product. The maximum size drive is 3.5". At present, the largest CAPACITY drive is 1 TB. But that will change as larger drives become available.
There are about 50 posts in the 'Upgrading the drive' thread you quoted. Several people have confirmed this is possible. Do you think they would be doing this if the additional capacity was not recognized?
As far as the rest of the questions, there are several links to another site in the 'upgrading the drive' thread. Try there.
There's a difference between needing help, and just being plain ole' lazy.
"You cannot teach a man anything. You can only help him find it for himself." Galileo Galilei (1564-1642)
HR20-700 with 2 TB, HR22-100, HR22-100, HR22-100, HR23-100 all running 0x5cd and networked.