Re: Issue with booting Ubuntu 12.04 from external hard drive
For many computers regardless of what you set for boot device order, if you want to boot from a device that is not always connected (or something like CD or DVD), you need to press a hotkey during BIOS splash screen (often F12, sometimes Esc or some other key) to select which device to boot at that time. That is probably a safety feature so you do not accidentally boot something potentially malicious. The only virus I ever caught was a boot sector virus spread by floppy disks a long time ago.
If you want grub to always boot whether the external drive is connected or not, grub and a /boot partition (containing /boot/grub) would need to be on your internal hard drive. Grub can be put in the mbr or on a primary partition (flagged as the boot partition if using Windows mbr), although, it may complain and need to be forced to install the grub boot loader on a partition. The mbr or beginning of a partition is not big enough for all of grub and its configuration files, which is why it needs /boot/grub whenever it boots, and that would not work if /boot/grub is on the external drive when it is not connected.
There are 2 potential issues having grub in the primary mbr when you have Windows. Some Windows programs may store data in what they think is an unused part of the mbr, but grub2 is somewhat larger than Windows mbr, so they can possibly store data within grub and make it not work (had that issue with Dell DataSafe). And some Windows service pack updates may refuse to install if Windows is not in control of the mbr (had that issue with a Win7 service pack).
So on sda I actually have grub2 on sda4 which contains 64-bit 12.04. But I normally boot everything from 11.04's grub in mbr of sdb (which is an SSD containing 64-bit 11.04). In a pinch, I could either boot sda which with Windows mbr would boot right to Win7, or if I mark sda4 as boot partition, I could boot grub on that partition.
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