I'm running Elementary OS on my Macbook from an external USB drive but I thought about installing it on a thumbdrive. Is there a way to install Elementary OS from my current install so that I can just run it (while booted into EOS) rather than creating a boot drive from the .ISO etc?
The only option that comes to my mind is using the UNetbootin
Hard Disk Install Mode. You can find the details here on UNetbootin Sourceforge page. I would also refer you to this AskUbuntu question for step by step process.
I think the hard disk here can be an empty partition on hard disk on the mac book or a new partition on your external usb installation, if you don't want to mess with your mac book hard disk.
Latest UNetbootin can be installed via following commands,
sudo add-apt-repository ppa:gezakovacs/ppa sudo apt-get update sudo apt-get install unetbootin