This message appears near the beginning of rebooting, immediately after a Boot device: ... message. Then, the system hangs. The problem is conflicting SCSI targets for a non-boot device. Having an external device turned off is unlikely to cause this problem.
For a solution, refer to "Boot device: /iommu/sbus/directory/directory/sd@3,0".
For more information, see the section on halting and booting in the System Administration Guide, Volume 1.