I have a Compaq iPaq, a very simple small form factor socket-370 i810 machine. I think the model is called "C500", which refers to the original Celeron 500MHz CPU (Mendocino). Mine has the legacy ports intact, it is not the "legacy free" version. I'll mention that the CPU has been replaced with a Coppermine P3 running at 550/100, but I doubt that matters.

The problem is it's a snob and won't boot any hard drive that didn't originally come with a Compaq. As you can imagine, this frustrates me.
It originally came with a Seagate Medalist 3.2GB hard drive. The system will boot from that drive, but I want to switch it out.
It will also boot from a Western Digital 10GB AC310000, which came from a Compaq Deskpro EN.
What I've found though is that it won't boot from anything else. The above 2 are the only hard drives it will work with.

I've tried several other drives. These are what I remember for sure:
Seagate Barracuda ATA IV 20GB
Maxtor 6.4GB with DOS installed
Hitachi Travelstar 12GB 2.5" PATA connected with an adapter which is known to work.
Western Digital ~6GB or so drive, don't remember the size or model

None of these will boot. The BIOS detects the drive, sometimes detecting the size correctly, sometimes not, depending which drive is being used. You push 'F1' and it saves the changes. It then tries to boot, but this ends with the message
"Attempting to boot from hard disk (C:)"
and it sits there forever. I think there is a blinking cursor.

These disks had a known bootable operating system on them. I believe I also tried booting a Windows CD and doing the install directly on the iPaq, and still had the same result when it rebooted.

I used a Hitachi drive feature utility to resize the Travelstar. I tried setting it to 8GB (using a preset in the utility). This didn't fix it. I then tried resizing it to *exactly* the same size as the original Seagate Medalist 3.2GB drive. The number of sectors was set exactly the same.
I used linux 'dd' to clone the original hard drive image to the resized Travelstar. At this point the Travelstar had exactly the same size and the same data contents as the original drive.
It still wouldn't boot. BIOS stops at the same message as above. I then tried the Travelstar in another machine and confirmed that it will boot elsewhere, just not in the Compaq.

I tried a Compaq "SmartStart" CD that I once needed for installation on an old ProLiant, but it refused to run on this machine. I found the support page for the iPaq on HPaq's web site, and did not find any hard drive "preparation" utilities listed.

I know some Compaqs have some BIOS features (like the configuration menu) stored in the hard drive, but I feel confident this iPaq does not work that way. The support site showed no utilities related to such an issue. Partition editing utilities show no sign of any extra partitions on the original drive. The BIOS has all the same configuration options regardless of whether a hard drive is connected. It's just strangely picky about what hard drives it will boot into.

Also note that some of my attempts involved exact byte-for-byte copies of the original hard disk, so any "hidden" data in the addressable storage area should have also been replicated.

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Does anybody know what's going on here? Did Compaq "sign" their hard drives in some way such that only their drives would work? Is there a way around this?
To reiterate, not only does the original drive work, but also a drive from a Compaq Deskpro EN works. That's why I'm wondering if they had some funky way of "signing" their drives.