What does probe-scsi-all show you at the ok prompt?
ok setenv auto-boot? false
ok reset
ok probe-scsi-all
The onboard SCSI port is Single Ended. If your drive(s) are Differential SCSI then they will not work. Also ensure that external targets are set to 4 & 5 just to be safe. Additionally, have you done a drvconfig;tapes to build paths to these drives?