External HDD won't restart on boot-up
 

I'm using a Dell Dimension 4550 (2.86 GHz, 1GB RAM, 80GB int HDD) with an attached external (usb) HDD.
The computer does not recognise the usb HDD as bootable but I can easily boot to various Linux distros installed on the usb HDD by means of a very simple boot CD.
However, my attempts to get Mandriva to boot from the external drive have not been successful to date despite an intense effort.
After some help I got from the Linux General section of this forum, I believe I now have a initrd.img image that does what I want it to do. Nevertheless, Mandriva still won't boot.
When the boot fails, this is what I see on the screen:
The problem, I believe, is that the fs is unmountable simply because the drive (usb HDD) where the root is has not been switched on during the formation of the initial ramdisk.
When I try this technique (booting to a Linux OS on an external HDD when computer BIOS doesn't have an option for usb booting) the external drive switches off immediately the boot CD starts to be read.
However, it almost immediately is switched on again (maximum 4 seconds later) so that this drive is up and running when the bootup wants to mount the root filesystem.
When I try this with Mandriva, however, the usb drive switches off immediately the boot CD starts to be read as normal, BUT it stays off.
Another thing I find strange is that the Kernel Panic message refers to the root fs is at (0,0)
It is actually on sda10 (on the usb HDD) which I would have thought would be (1,9)
I would welcome any thoughts
Thanks
Paul