I found out about this when I did it from http://www.tuxgarage.com/2011/07/add-virtual-ram-to-ubuntu-using-spare.html
Plug-in your USB drive and make sure there are no important files on the drive. Delete all the files on the drive or re-format it and go to a Terminal.
First, we need to unmount the drive if Ubuntu auto-mounted it when you plugged it in:
sudo umount /media/drive-label
Where 'drive-label' is the name you see for the USB drive when it is mounted.
Or you can also do it this way:
sudo umount /dev/sdx1
Where 'sdx1' is your drive/partition letter. You can find it by running this command:
sudo fdisk -l
Now, we'll create the Swap file on your USB drive:
sudo mkswap /dev/sdx1
Where 'sdx1' is your drive letter we figured out from the output of 'fdisk -l' command above.
Now, turn on your new Swap:
sudo swapon -p 32767 /dev/sdx1
In order to make sure your new Swap is working, you can take a look at the output of this command:
My output is:
/dev/sda2 partition 2047992 60692 -1
/dev/sdb1 partition 7912708 17764 32767