Have you tried starting up from cold with the drive inserted?

Being a spinning drive, it might be a problem with power - the PCMCIA slot might not supply enough to spin it up. I've used one on a Pismo via USB, but not via PCMCIA.

Are the ST1s you have all known-good? Got someplace else you could test them, like in another PCMCIA machine, an IDE-CF adapter, or a USB CF reader?

It might be necessary to format the drive elsewhere too. I recall some tales of having to format a CF on a PC, then erase & reformat it on the Mac.

Definitely shouldn't need drivers.