I just have the 1 Hitachi 1TB drive. I had it running external and then moved it to internal.
I paid about $300 from Newegg.
It works great and is a fast drive.
Absolutely possible, external eSata or replace the internal with larger hard drive. I'm using Hitachi 1T drive on my Hr21-700.
Ran it external for a while and the installed internal to get rid of the wires.