It is possible to make a "persistent Ubuntu" using unetbootin, but I haven't had much luck with it.
You can install Ubuntu on a flash drive just like you install it on a hard drive. Just boot from that bootable Ubuntu flash drive, plug in a secong flash drive, and install Ubuntu to it rather than to your hard drive. However, there are a couple of down sides.
- It will be very slow, because USB flash drives are slow. (USB3 might be better.)
- It will not be very portable. When you perform the installation, configuration files get set up for the target system. Trying to boot another radically different computer from that USB drive will likely lead to problems.
I don't know what your needs are, but every time I've tried this, I ended up spending a lot of effort for something that just can't be a solution to any of my problems.
