Open a terminal and type:
This will list all your connected devices. Look for your USB drive containing your swap space. There will be a UUID="XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX" entry (universally unique identifier for a device). Note the number between the quotes "".
gksudo gedit /etc/default/grub
Search for a line that begins with
and enter after the =
with the quotes! The Xs represent the number you noted after typing
blkid, insert them instead of the Xs. If there already is an entry of some kind, be sure to include it in between the quotes, too.
(For example, your line might look like this:
But do not simply copy and paste this, yours should look different!)
Save and close the file and type
This will take a small time. After it's finished, reboot. You should be all set now.