[jan-jaap]

IIRC I once tried to install a Cheetah 15K.3 or 15K.4 in a Crimson and that didn't work either. SCA model with an adapter.
I don't think it's a size problem. It may have to do with the way the adapter terminates 8 of the 16 data bits of a wide-SCSI bus, or maybe there IO board is simply not completely standards compliant.

My Crimson has a 'real' 50pin 18GB Seagate harddisk. They were already hard to find / expensive 10 years ago, I imagine a working one is proper unobtanium these days.