All sorts of options have been tried..
The referenced thread in the Information subforum, talks a lot about what people have tried.
A standard 500gb eSATA solution is going to set you back about $300 (roughly).
To start getting bigger then that, you are looking at RAID solutions and other items.
I have been chatting with a 3rd party vendor (not affiliated at all with DirecTV). They have been working on a product, that may be very intresting to HR20 users.
If you can make it through the rest of the TV Season with the current system. I would wait a little while before making a big $$$ invesment.