How do I make an external USB bootable? My main HD has 222GB in use, and is formated OS X 10.10 Yosemite (latest). I am marrried to 10.10, no time to upgrade.  I bought a 256GB (244 reported) USB stick (Sandisk midrange, their $56 midrange) and would like to image my whole hard drive to it, and be right up in my environment. 

How do I make this bootable and clone my HD to it?

To not "XY the problem", I want to finish with a bootable USB stick that is a clone of my current operating environment and can boot any iMac of my same model. 

Where I'm stuck, particularly, is "blessing" this USB stick to make it bootable. I.E. I don't know how to do MS-DOS 3.3's "format A: /S". 

What I've tried

I only install software from the app store. 

I tried booting up in 10.11 El Cap (external drive).  Its new Disk Utility "restore" blew an error "Not enough free space".  (300GB total, 222GB used, 244GB on USB). 

Unable to boot in 10.10 Yosemite since only the internal HD has a 10.10 image, and that's the one I'm trying to copy!  10.10 disk utility won't "restore" from the boot drive.  

Buying a 512GB stick is not an option.  

Resources at my disposal:

App Store.  Not afraid of Terminal.  

Noooo!  My copy of Install OS X Yosemite is  corrupted.  I store it as a tarball to keep it from exploding and self-deleting, and 'tar xzvf ' is giving a corrupted result. 

I am fairly adept with rsync (as local backup) and do my regular disk-disk backups that way, works slick.  

I also have a bootable 10.6 (Snow Leopard) volume.  Is it new enough to do this copy?
If you have previously downloaded earlier OS X installers via the App Store you can download them again (you will likely be warned that this version is older than the copy installed but you can continue with the download). Then you can use the appropriate commands to both erase and copy the installer files to your USB drive from https://support.apple.com/en-gb/HT201372