Emergency Bootable USB:  How to Create It:

1.  Identify a "live" operating system or other bootable package that you want to use as your emergency boot USB.  It needs to be in the format of an "ISO" file (name.iso), and should be less than 4G in size.  The size of the software on the ISO needs to be less than the capacity of your USB stick.  Usually, there will be some guidance online.  In the same place where you download the iso file, look for instructions about how much capacity will be required on your USB.*

2.  Find an empty USB stick that can hold the emergency boot disk.  Use a USB stick 4GB or smaller, since some OS will not be able to find the Master Book Record from a larger one.

3.  Put the USB device into your Linux computer and identify the path to the device.  Here are some ways to identify the device:
dmesg |tail  (should show last device attached)
sudo fdisk -l  (should show complete device path/name of last attached device)

4. Do not mount the USB stick.  If it mounted automatically, unmount it with the umount command:
umount <path to usb device>
example:
umount /dev/sdg

5.  Use the Gnome Disk Utility (gnome-disk-utility).  If it is not already installed on your system, install it.

In Filesystem (GUI) right-click the iso and select "Open with"
In "Open with" menu select "Open with Disk Image Writer" (this is Grub Disk Utility) 
It will open a GUI window.
Restore Disk Image
 Select USB destination
 Click "Start Restoring"
 
Are you sure you want to write the disk image to this device? 
  Click "Restore" 
 
Prompted for Root password 
Then "OK" 
 
See progress bar on top. 
Progress bar disappears when writing is complete. 
Box below progress bar shows how much of disk has been taken with image  
plus free space remaining. 
 
OK to remove USB stick since it was never mounted 

* The operating systems (or utilities) best suited for making emergency boot USBs are sometimes called "Live Linux".  If you search on the web, you will find several sites that recommend different OS for this.  I like Linux LiveMint, but my favorite has changed in the past and may change again in the future, since new software is always being created.