Complicated one this.
Drives: HDa is 40Gb with Win2k and Ununtu 11.10
One SCSI drive set in a raid array of one, to accelerate software loading on HDa
SCa: 80Gb with recently installed slack 13.1.

I had been using Ubuntu with the Grub loader and it worked fine.
Loaded Slack using two CDs which I had downloaded sometime ago. I printed off the chapter 5 of the slack install basics and followed the instructions. The software said it had loaded and I needed to reboot.
On rebooting the error messages was:
----------------------------
error: file not found
grub rescue:
----------------------------
And it hung.
So I am stumped.

In the BIOS the SCSI disc is not shown, only HDDs.

Using the Ubuntu live CD and Gparted
HDa is not shown, SDa is shown as 38.29Gb.
There are four partitions listed:
SDa1 ext 4 4.66Gb
SDa2 ext 4 33.63Gb
SDa5 ext 4 956Mb
SDa6 unknown 32.69Gb

Subtracting 4.66Gb and 956Mb from 38.29=32.67Gb, same as SDa6.

When I partitioned SDa I made:
/ 5Gb and bootable for the system files
swap was 1Gb
and the rest was for /home.

I cannot make a ubuntu boot disc as I cannot toggle to use the CDR, the live CD only recognises the CD in the DVD player.

Any ideas please.

Does the SCSI controller have a 'real' hardware RAID controller? In a "real" RAID controller, the drives are invisible to the OS. The OS sees the RAID. It sounds like you have a SCSI controller that does not have BIOS level hooks to boot. Is this correct?

Attached to the SCSI controller is an 80GB drive. Right?
Attached to another BIOS recognized controller is a 40GB drive. Right?

http://www.supergrubdisk.org/ Be aware that the power of supergrubdisk really doesn't begin to be understood until you get a good understanding a Linux boot process and GRUB. I learned quite a bit sorting out a problem very similar to yours.

SystemRescueCD is easier to get a system to boot... IF it can sort out your partitions.

It sounds to me like the boot partition is set to the SCSI drive and for whatever reason the root of that drive can't be loaded.