It is a very simple matter to change the HDD on your dvd recorder... naturally tho you will lose any information you have on the old and new disks, so I would HIGHLY advise backing up to dvd or something suitable.
The DVD recorders take IDE drives and they're currently up to just over 1TB, so more space is not a problem...
Just please bear in mind that changing the hard drive will most definitely invalidate your warranty...
The RH399H is a wonderful unit, although the performance of the front USB port is a downer, being only USB 1.1, but it can still play movies, music and pictures from the USB port, it just takes a while to get going.
I've had mine since july and have thrown 90% of the movies on my pc onto it and there's still like 200GB free... so the drive is more than capable.
The device also has the quickest startup of the one's i've seen, my dad's one take just over 6 minutes from pressing the button to usable.... whereas mine does the same thing in under 2 minutes...
All they are really missing from these devices is Ethernet or WLAN capability...