Not familiar with these ones, but I've dipped toes into the weirdness of pen computers from time to time, so I'll throw in a hint when I can.

I would tend to assume that the BIOS is only going to like up to ~8GB HDD in this, due to it's era, and if the 20GB can be got going it's likely to need overlay software to use all of it.

As it is vehicle mount also, some of the sites about fixing up old police and emergency vehicles may have info. There was one machine I went down a rabbithole with that that was where I was getting the best info.

Great tips! Yes, it is possible that someone installed the 20GB HDD expecting it to simply work.

Step 1 is getting a PS2 keyboard wired in as I will not have access to the BIOS or anything else without it.

I found a vehicle mount for the Hammerhead that looks like the same connector and screw post positions. It is just a pass through (brings the docking connector to another point) but it will be a source of the connector and I might get lucky and it will have some breakout points to rig up my own dock.

You made the correct call about the 8GB limit. I managed to get the drive to read with a USB adapter and found that it was partitioned to 8GB.

Thankfully, it appears that I was able to pull the data, but scandisk found about 7mb of broken file chains. Hopefully just windows stuff and not the rare stuff.

I finally received the vehicle mount and it has a PS2 connector which I couldn't see from the auction photos. The docking connector is also a nice ribbon cable with 2.54mm headers which makes it convenient for making my own dock.

But the big reveal is that now I know what the mating connector part number is: 3M 10150

These two part numbers are the solder cup version of the connector and the cable hood: 10150-3000PE & 10350-52A0-008 ($14 from China.)

Well that should make things easier, sweet.

Couple of things now that I can get into the BIOS setup:

1. This model does have USB. It is High Speed according to the bios. The vehicle mount has a USB port as well.
2. There is something more involved with my HDD issue.

The HDD that was installed worked in my USB reader. I tried it again and the tablet did not recognize a hard drive. I swapped in a compactflash-IDE drive and it does not see that either.
I am waiting on a PCMCIA floppy drive to arrive, so I have no options for booting at the moment.
I found a stylus which will be coming this week.
If I need to work on the mainboard, I am going to need to add a VGA connector to the dock because you really can't get to it with the LCD attached.

It is not clear from the BIOS if it is possible to boot from a USB stick. I might give that a try later today when I have some free time. If not, I am going to have to put it aside again until more parts arrive.

Did it like a regular stick? Gut feeling is it's gonna want under 8GB on a USB stick and legacy USB boot modes tend to be specific, have to set USB HDD, which a stick of an acceptable size to HDD support in the BIOS will usually work with, USB CDROM, which is actual external CDROM on USB, or USB Floppy, which can be a real drive. However, Rufus and maybe some other utils can format a USB stick to a USB floppy format, which some motherboards needed for BIOS bootblock recovery. Might need a 2GB or smaller stick for it. Now with a "floppy boot partition" on the drive, because floppies use FAT12, you can get it up to the max size for FAT12 which is 32Mb, which is a bit more useful than 1.44, but gets you a bit more space for troubleshooting utils and stuff to sort it out to boot by itself.

I actually haven't gotten back to this one yet. I got derailed working on the Libretto 20CT. You are probably right about the USB. It may even be specifically limited. The BIOS mentioned "Full speed" so it is very likely just USB 1.0. Originally it also had Windows 95.

But it is better than not having USB at all.

I actually have some period flash drives somewhere (like 128MB) "pendrives".