Emergency Bootable USB:  How to Use It

1.  If you can possibly do so in advance, find out what key or keys you will need to press during the boot process to gain access to the "boot priority menu" of the machine in need of repair.  You should be able to find this online by searching for the make and model of your machine, plus the words "select boot options".

If you can't do this in advance, you will need to watch the boot process very carefully to get the information, and then you will need to be quick with your fingers to press the right key to get into the boot menu.

2.  BEFORE booting (or rebooting) the broken machine, put the repair USB stick into a USB slot on the broken machine.  If you do not do this first, the option will not show up in the boot menu.  

3.  Boot the broken machine and press the key or key combination for the boot menu immediately after seeing the instruction to do so on the monitor.

4.  In the Boot menu, make your USB drive the primary boot device and save your changes.

5.  Boot into the USB device, you "rescue" operating system.  Confirm that you have successfully booted into the operating system of your rescue devie and not into the operating system of your broken machine. Either the commands "hostname" or "df -h" should give you a clue.

6.  Mount the broken machine with a "writable" option so that you can make whatever changes are necessary to fix the device.  If you have an easy emergency repair operating system installed, this might be doable either through a menu item or the GUI.  If not, use "fdisk -l" to view available hard drives.  Look at the first column of the fdisk output to find the device path to your target hard drive.

7.  What happens next depends on how you have created your emergency boot disk and what is wrong with your broken machine.  In general, you should be able to mount your target hard drive, apply the required fix and save it, then reboot the system and early in the reboot process, remove the USB stick from your machine. The first part should look like this:
mount <device name of broken machine> <empty directory or partition on USB stick>
Also helpful:
sudo mount -o remount,rw /
sudo cp <filename1> <filename2>

* This won't work for fixing the broken computer if the broken computer contains a "software raid."