To add a second disk to a machine in SCO use the mkdev hd command.
This is the same whether scsi or ide
Connect the second drive and note its scsi/ide adapter, ID etc.
login as root
run mkdev hd
Enter the details as prompted for the disk
It will need to relink the kernel
do this and reboot
Run mkdev hd again and enter the same details
It will then mount the disk
You then need to divvy the second disk, create filesystems and mount them.
divvy diskname
Put names on the filesystems that you see listed (if there are ones there already - DO NOT CREATE filesystems if there are already ones in there as this will erase everything on the disk)
If its a blank disk, create the filesystems using start and end points and name and create them.
Then run mkdev fs to mount the named filesystems
Choose the disk partition name and then the filesystem name you are giving it.
Finally - mount the filesystem
mount /dev/filesysname /mountpoint name
e.g. mount /dev/u2 /u2
As you can see, it is a complicated long-winded job and you need to be careful running it. I have done it lots of times and had no problems as long as you remember the important rules -
Only choose to create a filesystem while in divvy if there is nothing in it
Make sure you have all of the scsi/ide adapter and ID info
Post any further queries and I'll help if i can
good luck
Steve