Are you sure your drives have a SCSI interface. ATAPI drives are IDE drives. have you gotten these details from a computer scanning program?
Things to try,
Unplug the IDE or SCSI interface lead from the rear of one of the drives, when the computer is switched off and try the other drive. Do this for both drives.
Have the drives worked with no problems before? have you checked the jumper settings of the drives. They should be master and slave. Or both set to cable select.
Are the data leads pushed firmly into the rear of the drives and into the motherboard.
have you tried another data lead. it could be that yours is damaged.