The T5100 as other similar IDE Toshiba laptops has a handful of hardcoded supported hard disks, in this case a set of four hard coded types and corresponding set of Cylinders, Heads, Sectors parameters the latter of which start at offset 0xE451 (while the strings to show the capacity start at 0x4555)

1Type 6 (40MB):	2503 04 0000FFFF0000000000 2503 1A 00
2Type 7 (40MB):	D303 05 0000FFFF0000000000 D303 11 00
3Type 8 (48MB):	6502 06 0000FFFF0000000000 9E02 1A 00
4Type 9 (100MB):	0803 08 0000FFFF0000000000 0803 21 00

The format is cylinders (inverted) heads *some other stuff* cylinders (inverted) sectors *unknown byte*
So for example the first is for a drive with 0x0325 (805) cylinders, 4 heads and 0x1A (26) sectors.

To add support for another drive or CF card it's necessary to first merge the EVEN/ODD binaries which can be done with an utility such as romwak :

To merge:

1romwak /m T5100_042E_V2.20_EVEN.BIN T5100_043E_V2.20_ODD.BIN T5100_V2.20.BIN

To split:

1romwak /b T5100_V2.20_64_512_CACTUS.BIN T5100_V2.20_mod_E.bin T5100_V2.20_mod_O.bin

After the merge simply replace a set of parameters with the ones of the new drive or CF card, adjust the capacity that's shown in the configuration menu and fix the checksum by adjusting the very last byte (the CRC-8 of the whole document must be 00) which can be done with HxD.

Unfortunately this Toshiba seems super picky with the kind of CF card it'll actually work (as far as I can tell there are no additional hard disk checks, but I might be wrong) so YMMV.

The only CF card that finally actually worked for me is a 512MB "Cactus Technologies" model KC512MF-303 with these parameters:

F203 10 0000FFFF0000000000 F203 3F 00

1010 cylinders (0x03F2 so F203)
16 heads (0x10)
63 sectors (0x3F)

The adapter I used has a 40 pin male header and molex power connector (so not floppy) so it's pretty much a drop in replacement, I will position it so it's accessible from the expansion slot on the back.

I'm attaching the original ROM dump and the modified one, both the merged binaries and the EVEN/ODD split. The modified version also contains the parameters for a 64MB Sandisk card in place of the 48MB disk which didn't work but I forgot to remove.

Related: the very similar modification I had previously done to my T3100SX:
Toshiba T3100SX Modified BIOS / Help with its IDE