I have two ubuntu 12.04 installs on a remote powered terrabyte external hardrive connected to the PC via USB.
The pc I am currently on also has two Edubuntu installs but that point is moot (I hope) as my goal is to have the external drive boot regardless of what PC it is connected to whether it be Windows, Mac, Linux etc (and assuming its bios can be set to boot to USB of course).
I messed up Grub from the beginning by temporarily replacing the PCs hard drive with the external one (I.e. making it the main internal one) because installing the two OSs via sata was so much faster than usb. When I returned everything to normal (old hard drive back in and the external to USB) then I couldn't boot the usb drive.
I've tried a couple command line utilities (boot-repair is one I think and the other is mbr) and got nothing but I can boot into them using Super Grub Disk2 by enabling the usb functionality (that worked like a charm by the way even if it still experimental). The site I used for instructions for trying mbr was http://www.pendrivelinux.com/install-a-new-mbr-to-your-usb-flash-device/
Open a terminal and type sudo su
Type apt-get install mbr
Type fdisk -l to find out which device is your flash drive
Type install-mbr /dev/sdx (replacing x with your flash device)
I typed install-mbr /dev/sdb
So, what I want to do is rewrite the external harddrive so that when I set the bios to boot to it it will work.
Running Super Grub Disk 2 without the usb enabled shows the two partitions on the main hard drive. After enabling USb functionality it shows all four installations and the external drive is showing as sdb.
If I set the bios to boot to the USB I get a black screen and error - MBR 1FA
I'm also trying to understand the hard drive naming when the PC boots to the external. I'm assuming the external then becomes sda but I'm not sure.
Any help with these issues would be greatly appreciated.