boot existing sda1 from USB

Question

I can manually boot my sole Ubuntu 10.10 install from my USB stick typing the following commands:

set root=(hda0)
linux /vmlinux root=/dev/sda1 ro
initrd /initrd.img
boot

What I was hoping for is to have the USB do it for me. No typing required...?

I followed directions from these urls (most didn't work)

grub on a bootable USB rescue stick

How to install GRUB 2 to USB drive / pendrive

GRUB2 and USB flash drives

How To Recover GRUB2 Linux

and some pretty extensive searching here. There's 10 pages at 50 per page. I did what I could and now turn to the experts.

Summary of steps used to make USB boot to grub2, so far...

mkdir -p /media/GRUB2_RESCU/boot/grub
sudo cp /usr/lib/grub/i386-pc/* /media/GRUB2_RESCU/boot/grub
echo '(hd0) /dev/sda' > /media/GRUB2_RESCU/boot/grubdevice.map
sudo grub-install --root-directory=/media /dev/sdb
sudo grub-mkconfig -o /media/GRUB2_RESCU/boot/grub/grub.cfg

Thank you for your time.

Answer 1

The steps you gave are close to working, but let me comment on each step.

mkdir -p /media/GRUB2_RESCU/boot/grub

This is completely correct.

sudo cp /usr/lib/grub/i386-pc/* /media/GRUB2_RESCU/boot/grub

This isn't needed but shouldn't hurt anything (grub-install does this automatically)

echo '(hd0) /dev/sda' > /media/GRUB2_RESCU/boot/grubdevice.map

This also isn't needed, and also shouldn't hurt anything. GRUB2 for the most part doesn't need a device.map anymore.

sudo grub-install --root-directory=/media /dev/sdb

I don't know if this was just a typo when you were writing your question or if this is actually the command you ran, but it should be "--root-directory /media/GRUB2_RESCU/". Otherwise grub will install its modules, and look for its grub.cfg, in /media/boot/grub (which isn't on the flash drive) instead of /media/GRUB2_RESCU/boot/grub/. This could explain why what you tried didn't work.

sudo grub-mkconfig -o /media/GRUB2_RESCU/boot/grub/grub.cfg

This should work, but you'll have to run grub-mkconfig manually every time there is a kernel upgrade to get an updated menu. Instead I would recommend writing a manual grub.cfg which just loads the grub.cfg from your internal drive, like this:

root=(hd0,1)
configfile /boot/grub/grub.cfg

To be sure that this works even when drives are moved around replace

root=(hd0,1)

with

search --set --fs-uuid UUID_GOES_HERE

You can find the UUID with this terminal command:

sudo grub-probe --target=fs_uuid --device /dev/sda1