Just to touch base, in case anyone is wondering, I'm trying to swap HDD's with my Toshiba RD-KX50 80 GB HDD/DVD-R/RAM unit.
My unit's specs are as follows:
Unit Software Version: YA10 (Y-A-one-zero)
DVD Drive Software: J130 (J-one-three-zero)
HDD Specs:
Seagate
U Series 9
80 Gbytes
Model: ST380012ACE
Firmware: 9.51
Site Code: WU
I found a 160 GB Seagate drive, model ST3160023A, but it's a bad drive. The KX50 got to 10% format, then spat out "Unable to format DVD." Interesting that it spat out "DVD."
Another thing: I fired up the original HDD with the jumper set on MASTER, and it booted and operated normally.
I tried a WD3200JB, with the jumper set to master, and it did the same thing as the other WD drive.
I'm tempted to try booting up one of the WD drives, and when the error code pops up, try hot-swapping the original HDD back in. I'm not ready to try that just yet, though. Reason #1 being that I still have content on the original HDD (playing with fire, I know), and secondly, I found an ST380012ACE on eBay for $37.90 altogether.
I want to try what rickc5 has suggested about getting as close to the original HDD's specs as possible, and I think this is as close as it gets (without dropping a grand on that bogus site a few posts back

)
The specs on the new drive are:
Model: ST380012ACE
Part Number: 9W6022-001
Firmware: 4.06
Site Code: WU
In the tiny picture that follows the information, I can even make out the message about the drive being OEM, just like how mine does. I'll keep my fingers crossed and report back with any new findings. Hope this is helping somebody out there.
Ben