Correction to the last reply: everything is right on the money except that, with the dual channel Symbios SCSI integrated controller, you can use SCSI B to create an external SCSI port. So if you do not have a NetRAID card installed, then your hot swap drives are cabled to SCSI A. This leaves SCSI B free to add the internal to external SCSI cable.
The primary configuration where both SCSI A & SCSI B are used would be when both hot swap and common tray hard drives are installed. If both integrated SCSI ports are already in use, you will need to add a HBA card.