Hello I managed to successfully modify the bios of my Toshiba T3100SX to change the hdd parameters so I can use whatever I want. I prefer to use CF cards whenever I can but I'm having issues with this Toshiba.
The IDE connector is not standard, is a 44 pin but with the 40 pin pin pitch (if that makes sense) this is not really an issue since is possible to use a normal 40 pin cable/adaptor and skip the extra 4 power pins.

To change the parameters these are the two areas I modified corresponding to the Normal and Alternate options originally for the 80MB HDD (the only two I could chose in the bios settings). The other parameters are in the same general area easily noticeable.

0x1E411: 5E 03 04 00 00 FF FF 00 00 00 00 00 5E 03 30 (Normal 80MB)
0x1E491: B7 02 0E 00 00 FF FF 00 00 00 00 00 B7 02 11 (Alternate 80MB)

bytes 1 and 2 are for the number of cylinders in reversed order. For example 862 cyl is 0x035E so you write them as 5E and 03 in the binary
byte 3 is the number of heads in hex as is
bytes 13 and 14 seems to be again the number of cylinders
byte 15 is the number of sectors in hex as is

Here's my changes
0x1E411: E2 03 08 00 00 FF FF 00 00 00 00 00 E2 03 20 (128MB CF)
0x1E491: DD 03 10 00 00 FF FF 00 00 00 00 00 DD 03 3F (512MB CF)
(I also changed the strings at 0x145AD and 0x145CD to reflect the new capacity in the menu)

To change the checksum, the last byte, I use Hex Workshop. When you generate the CRC 8 on the whole document the ouput should be 0 when the checksum is correct. Adjust the last byte until you get 0.

HDDID seems to agree with these parameters, but the HDFAT utility (launched from TESTCE31 when you chose to format the hd) shows different parameters.

Any help on how to properly make a cf card work with this machine will be appreciated. I'm attaching my original bios dump (version 004C) and my modified one.

UPDATE: Using another cf card I managed to install dos. However I still get an error saying that there are invalid cmos parameters (I can proceed to boot pressing F1). I suspect is because is somehow detecting different parameters for the cf card then what I used in the bios. I have to do more tests but for now the power supply board decided to give me headaches so I'm waiting the capacitors for it.