Yesterday, I had to rescue a broken Ubuntu 14.04 installation by booting from USB. Unfortunately, I was unable to get into the BIOS to change the boot order (because of a BIOS password and a bad memory).

Fortunately, since I was able to get to Grub, it was still possible. Here’s how I did it:

• Create a bootable USB drive (using something like Startup Disk Creator. Before taking the drive out, locate where the vmlinuz and initrd.* files are located. You’ll need them later.
• Insert the USB drive and boot the system. When you get to Grub, press c to get to the ‘command-line’ option.

Here is where it gets a bit tricky. In my case, I knew the root partition on the USB disk was /dev/sda1, yours may vary.

Since Grub uses a slightly different device mapper, let’s use it to find the partitions:

grub> ls
(hd0) (hd0,msdos5) (hd1) (hd1,msdos0)

This will show you the available devices. In my case, the relevant partition was (hd1,msdos1). Now, let’s use this information, along with our knowledge of where the vmlinuz and initrd files are to boot the system:

grub> linux (hd1,msdos1)/install/vmlinuz root=/dev/sdb1
grub> initrd (hd1,msdos1)/install/initrd.gz
grub> boot

That’s it. You should now be able to boot straight into Ubuntu. This should even work if your BIOS doesn’t support booting off of USB.