Recently I tried to install ubuntu. The install on an XP System and was to a USB disk drive (Not Flash) appeared to go well until I rebooted the system and the system locked up with error 21 (Disk not found) No windows nothing nice one Ubuntu? I went into the bios turned off the disk rebooted the cd into Linux and looked for a method to repair the MBR in Ubuntu I could not find one, but Mandriva did offer to fix this. I now have Windows back (and can write this). Ubuntu, although I think it is all there, won't Boot. Is there a quick way to get the ubuntu to boot?

The quickest way would be go into your bios and change the boot order to usb first. If Grub is installed in the Ubuntu partition it should boot.

I got error 21 at some point too. I don't remember how I fixed it, only that it was something simple and a quick web search turned up the answer. There are several suggestions here that might be helpful:
http://ubuntuforums.org/showthread.php?t=24113

Thank you both for your input.
I think I may not have explained it well. The computer current tries to boot from USB first. I wanted to get a system that booted to windows unless I had the Linux USB drive plug in. When I ran the install I did not find any option not to installed grub Ubuntu just went ahead and did it, which would have been ok if it had worked. Now thanks to Mandriva, Grub is gone. If it was a version of windows on the USB drive my first try to fix it would be to run Fdisk and make the first partition to bootable does Linux have a similar system. I will fix grub if I can’t get this option to work.

Roger