My Ultra 5 also has the same issue if I don't use one of two specific driveis: The original BTC IDE drive or one specific HP IDE CD burner. All others throw that error, even SCSI ones.
My Suns have never been that picky, but then again, I have an older Plextor SCSI CD-ROM drive that seems to work with _everything_ I've ever thrown at it.
One thing I noticed after looking at the pics above is the device line for the CD-ROM is
sd@6,0:d
which means it's trying to boot from partition "d" on the device (other way to think of it might be partition or slice 4). If that's not the bootable partition on the disk, you would likely get that error. You might try sd@6,0:a
Found this Google book link where Chapter three is "Booting and hardware diagnostics". Perhaps that will help you.
I have been working on a few lately, and all of thew had the same weak points, those LCD capacitors leaking all around, the capacitors inside the PSU leaking also, and the casing fixing studs breaking away.
I have a Mesonic thing that vaguely resembles a dark green (a sort of cross between original bondi blue and lime) tray-loader that I got new (for a few dollars!) on a whim at a convenience store around 2001-2002.
Unfortunately, I've moved a bunch since then, and it got smashed up quite badly. I'd love to find a replacement.