The diskette has the device driver for the SCSI host adapter.
You should be able to make one from the Compaq/HP Smartstart CD.
You will also need the serial number and activation key.
Does this system have RAID?. It seems to me that a two drive system, with one of 4 to 8gb and one of 72gb is rather unusual.
More likely the system has two 72gb drives in it that are mirrored.
Some RAID systems will only boot from drive 0 of a RAID 1 configuration, so it may appear that all data has been lost even though only one of the two drives has failed.
I strongly suggest that you find a local SCO reseller, or at least a local HP dealer in order to determine the exact system configuration before you do anything.