[Sheflug] boot weirdness

[Richard Balthazor <R.Balthazor [at] xxxxxxxxxxxxxxx>]

I've a machine with an IDE primary master HDD (/dev/hda) (with Win XP), a
primary slave (CD-R), and secondary master HDD (/dev/hdc) (with Redhat).  
The dual boot is managed by GRUB; the boot loader is installed on the MBR
and the rest of it in /boot, as per default.  All works well.

I added a secondary slave HDD: set the jumper to 'slave', detected it in
BIOS with auto-detect and then switched to User-defined whilst keeping the
same settings, and plugged it in.  Now it reaches Grub and fails either
with "GRUB Hard disk error" and Grub error 21 (ie, semi-randomly).  The
secondary master HD definitely has the master/single drive jumper setting.  
The sec. slave HDD (which has a non-bootable filesystem on it) is
recognised quite happily as /dev/hdd (as expected) if I boot from CD, as
are all the other disks.  Running grub or grub-install manually from a CD
boot also recognises all the disks in the correct order (hd0=pri. master,
hd1=sec. master, hd2=sec. slave).   I'm getting a similar problem with
LILO but I'm not as familiar with that.

So, if I understand correctly, adding the secondary slave is confusing the
boot loader on the MBR sufficiently that it "can't see" the stage 2 boot
(which is on the sec. master) - either it suddenly can't see hdc, or it's
managed to swap hdc and hdd and is looking in the non-existent
/dev/hdd/boot, although booting from CD seems to deny this).   

The only really dumb thing I haven't tried is a) cable swap, or
b) swapping the order of the drives on the cable, which I haven't tried
through lack of cable.  Surely it can't be this?  

Any ideas greatly appreciated. 

cheers in advance,
R.

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