[AK5fa]

The fuel mainboard has a 68 pin connector for hard drives and digital audio tape and a 50 pin connector for the CD, both are SCSI.

Yes, the Fuel mainboard has a 68 pin connector for U160 SCSI drives. But it's a SCA68 Male connector, which is physically quite different from the HD68 female connector that are used on the HDDs and on the add-on Qlogic 12160 SCSI adapter card. I can post some pictures here if you need more detail. 
That's why any of the standard SCSI cables cannot be used to connect the Fuel mainboard to an HDD. You need a special cable or some kind of adaptor. As @weblacky mentioned, a Google search for IBM 40H7572 would give you an idea of what kind of cables/connectors are needed...

Thank you and Best Regards,
AK

You’re putting in more work than just buying the correct cable. If you have no other prom or L1 issues, then you’re doing great. Normally either you should have a bad power supply or you should have tons of prom errors due to faulty internal monitoring. So if you have none of those then just buying a cable gets you off easy when it comes to getting a fuel running.

Hello weblacky,

Thank you for a detailed reply and an offering me the proper SCSI cable (direct from mainboard to the HDD). I really need to think in that direction...

But what puzzles me the most in my current situation (with an add-on SCSI card connected to the HDD) is:

Why does the PROM refuses (can not?) see any data on the the dksc(2,1,0) or dksc(2,1,0) devices or even the devices themselves?


I have tried to replace the original SCSI cable which was included with the system with a new one which I have.  The results are the same for both HDDs!
As I mentioned in the OP, the HDDs are detected (the dksc(2,1,0) device and model name of the HDD are visible when I do "hinv" and "hinv -v" respectively),
but no additional data from the HDD can be seen (the "setpart -l" returns nothing)... Of course, the "boot -f ..." does not work either.

I doubt that data on 2 or more HDDs has been corrupted in the same way. And the problem with the SCSI cable seems unlikely. Then, could it be a problems of
the SCSI add-on card?

I do have at least 2 Adaptec 29160 PCI-X cards, but I did not try either of them.
Can those cards be used in the Fuel as is or they need to have a different Firmware / BIOS?

I am worried that if even if I go through the troubles and get the direct SCSI cable (from the Fuel mainboard to the HDD), I would end up facing some other (perhaps a BIGGER) problem. 
So, could you please recommend me to some other preliminary tests to see that the HDD data is more or less consistent with a working IRIX OS and file system?

Thank you and Best Regards,
AK