Well, it seems that not all SCSI 50-pin female to 68 pin female adapters are created equally.
I bought several of these from Amazon, made by Monoprice. (Pictures below.) My SCSI chain in the XL1201 was then:
SCSI on Backplane -> 50-68 adapter -> cable -> ST318451LW -> cable (same cable) -> terminator
The terminator is built into the cable and is labeled "Foxconn Ultra 320."
This was able to boot my SCSI2SD v5 card (with another 50-68 pin adapter) but not the ST318451LW.
I pulled out the 50-to-68 pin adapter from my MacIvory's CD-ROM (which DKS converted to 68-pin as well) and put that into the backplane SCSI port and voila - the ST318451LW would boot. Moreover, I could add the SCSI2SD to the chain and both would be accessible.
So, it seems that not all 50-to-68 pin SCSI adapters are created equally. (Either that or my broken SCSI Pin 1 is incompatible with the adapter.)
