Casper's suggestion worked for me, it was to add "set scsi_options=0x58"
inside of /etc/system.
This allowed me to be able to access the external ST15150N drive from
booting off of the internal disk, as well as accessing it while booting
from the CD-ROM.
This information has pointed me in the right direction to troubleshoot
this problem, and not waste a ton of time in the process.
Thanks!
-Brian