- Hard drive
- Removable drive
- Optical drive
I made a few other discoveries about booting from a USB memory stick. Ubuntu provides a USB Startup Disk Creator utility, but this works only for Ubuntu distributions. There is another utility called unetbootin which supports a much wider range of distributions and has options to select an arbitrary ISO CD image for its content - however, I wasn't able to boot from the USB stick when I tried this option. I've seen suggestions that it is not possible to boot an arbitrary ISO CD image from a USB stick without having some knowledge of what's inside. Based on my experience, being able to boot from a USB stick is not a complete replacement for having a bootable optical drive.
One last thing. If you use the USB Startup Disk Creator utility there is an option for storing documents and settings in "reserved extra space" (the default option), or discarding them on shutdown. I created a bootable USB memory stick and tried it on my laptop and it worked fine. When I transferred it to my new MythTV server, it failed to boot, coming up with a number of obscure disk encryption errors. It turns out that when I booted my laptop, it wrote my-laptop-specific configuration to the USB stick which then rendered it useless on any other machine. I'd strongly recommend using the "discard files on shutdown" option..!