I have formatted a 3.5" 20GB ide hard disk mounted in a USB caddy with seven partitions:

sda1: boot 32M
sda2: swap 512M
sda3: 2G
sda4 ext - sda5 1G; sda6 1G; sda7 12.5G

All save swap have been formatted ext2 and I want to try and make my thinkpad and dell be able to boot off this disk.

I tried to install grub to this device but it will not recognise the drive number

I tried things like

root (hd0,0)
install (sd0)

no go

root (hd1,0)
install (hd1)

no go ...

I wonder if this can actually be done this way - I have been reading a bit on how to make a USB boot disk and things still don't quite add up.

First step I would like is just to have grub command line so I can use this disk as a rescue and recovery disk - eventually with its own mini kernel and some tools etc...

Any tips?

Will

I have rebooted to sus out the BIOS options more fully and find that if I select the USB HDD option at boot I get a grub command line!

I will try too boot to my existing HDD partitions/root from that just to confirm that what I am trying to do is possible.

I got errors about drive number when I tried to install grub though - so I am confused - this drive was just formatted so there is no grub on it (surely it is not hung over from a previous partitioning and multiboot exercise - could the mbr still have that original grub installed on it?)

Because when the last install was on this drive it had been in a laptop.

It would seem that the ThinkPad has no trouble allowing booting from USB drives but will a kernel panic when it tries to load its own USB drivers (/??) much more reading I am guessing - anyone know a tried and trusted way to do this/
?

Will

Grub supports tab completition, so just type "root (" then <tab> - it'll tell you what it can see. Same applies to "setup", which I think you should be using instead of "install".
Quote from the grub manual re "install";
"This command is fairly complex, and you should not use this command unless you are familiar with GRUB. Use setup (see setup) instead."

As for booting a kernel off it, I'd guess you'd need an initrd/initram. Haven't done it, so hopeffully someone else can tell you for sure.

First you have to tell grub which device you want to work with::

grub> device (hd0) /dev/sda1

Then you can do root (....

And yes, syg00 is correct, use setup, not install.