What exactly is a “persistent” install?
With a normal Live-CD boot of Ubuntu, you can’t save your session settings on exit of the OS. And if you boot off USB stick from a transferred CD image (such as one created using the Unetbootin utility), that won’t save your settings either because the OS is still in Live mode on boot.
A persistent install to USB stick does allow you to save your live session settings.
I wanted to try out this whole persistent-install thing just to see how it worked. Here’s what I have to report.
Of the tutorials for persistent install how-tos on the internet, the ones on pendrivelinux.com work the best, no question. The tutorial I followed is USB Ubuntu 8.04.1 Persistent install from Live CD.
I will say up front that doing this requires a ton of command line stuff. There is absolutely no GUI involvement, no click’n'drag, none o’ that. Terminal is your friend on this one. 
If you follow the directions to the letter, do it slowly so you don’t skip anything, it does work.
Here’s what I have to report about my experience with a persistent USB install.
The good and the bad
Performance
Notably slower. Boot-up takes longer, shutdown takes longer, general operation is slower, etc. This is not a speedy way to run Ubuntu whatsoever. And even if you used a variant such as Kubuntu or Xubuntu, it’s still slow.
Does it actually save your settings?
Yes. For a test I configured a wireless connection and installed the Flash plugin in the Firefox browser. On reboot, it was all there when I got back into the OS. Very cool. It also saved all the other settings (for the window manager, fonts, etc.)
Is it secure?
No. You lose a huge chunk of Linux security when you boot this way. Because it’s still technically a live mode, the system asks for no password on login. In fact it doesn’t even ask you to login, Ubuntu just goes straight to the desktop.
If anyone got your USB stick and booted it, yes they would get to all your stuff. Easily.
Is this a true installation?
No. While it’s true your settings are saved and you can operate the system normally, a live mode is a live mode no matter how you use it. It’s called “live mode” for a reason (several reasons, actually).
Is it possible to install a full installation of Ubuntu on a USB stick?
As long as it’s over 2GB, yes. A 2GB stick is just a smidgen too small for a full install according to the Ubuntu installer (by literally a few MB). So if you grab a 4GB stick, yes, you can install a full version.
Yes there are ways to get a full install of Ubuntu on a 2GB stick, however you’d have to do it not using the native Ubuntu installer. In other words, it’d be a pain at best to get it working. Besides which, running a full CD-sized distro on 2GB just isn’t smart because you’re guaranteed to run out of room fast.
If you want a full install of Linux on 2GB-or-under sticks, consider Puppy Linux or Damn Small Linux, both of which can be pushed to USB stick with bootable ability via Unetbootin.
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