Hmm, if it's a grub shell then "mbr" isn't going to work.
From the GRUB manual:
How to uninstall GRUB from my hard disk drive?
There is no concept "uninstall" in boot loaders, because if you
"uninstall" a boot loader, an unbootable machine would simply
remain. So all you need to do is overwrite another boot loader you
like to your disk, that is, install the boot loader without
For example, if you want to install the boot loader for Windows,
just run `FDISK /MBR' on Windows. If you want to install LILO(1)
, run `/sbin/lilo' on GNU/Linux.