Ah, OK, so in step 6 you're talking about 8" disks, i.e. the other port/connector/cable?Re-read my corrected post. The system three only boots from the B drive.
Before you go to the trouble of changing the cable, why not try what I suggested and just move the drive select jumpers?
When you select drive B to try a DIR, are you selecting the correct drive size and type?