It couldn't have been easier!
After looking at Jeff's pictures, I decided to give it a shot. I already had a Western Digital 120 GB 5400 RPM drive that was a backup drive on my home network, formatted in NTFS. I took the cover off the HDR230 with no problem, disconnected the ribbon and power cords to the original drive and connected them to the 120 GB drive, then turned the power on expecting a lengthy delay for formatting or something. Nope! It just started right up and showed the last channel it had been tuned to. While fooling around with the remote, I got a prompt to reset the clock. So I reset it. Then I checked the "reservations list" and saw that it now says 12 hours 36 minutes available (originally it said 8 hours 14 minutes). [note: set the hard drive jumpers for "cable select"]
I've tried timeshift functions and I'm now doing a short timed recording. It appears to work like a champ. In a few minutes, I'll physically remove the original drive and secure the 120 GB drive. Why Zenith didn't just use a 120 GB drive, I don't know. For a thousand dollar machine, you might as well have as much capacity as possible.
I'll post further info if I run into any glitches. I'm going to save the original drive unchanged for a while.