... your system needs a SCSI adapter driver !!!
However SCSI basically is supported,
you need to tell the system that you are that specific Adaptec AHA-2940 unit and need to provide its drivers and the according entry in the CONFIG.SYS !!
After recognizing this SCSI-Addapter the Plextor, like here, will be identified without any problems !
I assume your 2nd unit too