I am guessing this is a SCSI tape drive and the controller is a SCSI controller which currently works fine with your boot drives.
It seems odd to me that your controller would make a difference here if it is already working for other devices (i.e. your hard drive array).
When you tried your mkdev, was it unable to find the device, or are you just unable to access the tape drive when you try to use it. I am guessing it was the former.
I presume your controller has software or firmware for managing the arrays. Are you able to see the tape drive from the within the controller software?
If no, your problem is between the controller and the drive, and is most likely related to cable termination or conflicting SCSI IDs. If yes, you may need to configure the controller so that SCO can see the drive as a device off of the bus.
Assuming the controller can see the drive, and there's nothing there to hide it from SCO, the next most likely hangup is mkdev. From past experience I know it can be kind of tricky getting the right values for the mkdev prompts, and if memory serves me correctly the prompts in mkdev are actually in a different order than expected.
Assuming the tape drive is on the same controller as your boot hard drive, you want to use controller 0, bus 0, ID 5, LUN 0. The drive is possibly on a different controller or bus, but the LUN should always be 0. Your chosen SCSI ID is unusual because SCSI ID 5 is typically used for SCSI CDroms, and ID 2 is used for tape drives. Do you have a SCSI CDRom set to ID 5? That may be the problem.