Booting problem

[Keith Powell]

My set-up here is two internal hard drives. One has XP on it and the 
other Ubuntu  8.04.1. Each installation is entirely separate. While the 
BIOS is booting, I select which hard drive to use.

This has been working perfectly until a couple of days ago. Now I am 
having a big problem!

With only the Ubuntu hard drive plugged in, it boots correctly.

However, with both hard drives plugged in, Ubuntu starts booting and 
then stops with the error message:

BusyBox v1.1.3 (Debian 1:1.1.3-5ubuntu12) Built-in shell (ash)
Enter 'help' for a list of built-in commands
(initramfs)_

I have tried rebooting in safe mode, but get the same message.
I have tried using the XP System restore and going back to an earlier 
point (in case there was an XP problem).
I have even reinstalled Ubuntu with the XP drive unplugged.

All are the same: Ubuntu is OK if XP is unplugged, no good if it's 
plugged in. For information, XP runs whether both drives are plugged in 
or if the Ubuntu is unplugged.

It's defeated me and I don't know what's causing it.

Any ideas, please? (Apart from "Don't use Windows!!)

Many thanks

Keith