GRUB from external HD not working

[elfrank]

I installed ubuntu in my external HD, it's in the partition #4 of the disk. The GRUB is already installed in the MBR but when the computer boots I get the command line of the GRUB (Grub>). I tried modifying the file menu.lst and I added the line root(hd0,4) where Ubuntu appears but it still doesn't work. Any ideas?

I suppose the solution is simple but I'm new on Linux.

Thanks.