If I remember correctly, these 68 and 80 pin drive have 16 SCSI IDs  (4 pins) while the Mac only has 7 (3 pins). Make sure that the 4th pin on the ID is not used on the 68/80 pin drive; it will not be recognized by the Mac and the drive will be waiting for commands that will never come for IDs 8 - 15.

ID #7 (all first three pins jumped) is for the Mac itself, everyone else's ID is 0 - 6.