What I'd like to do: have Ubuntu installed on a USB drive and from there install any ubuntu version on a hdd.

This is kinda different from using usb-creator because I'd like to have a persistent ubuntu install on the USB drive and not upgrade it every 6 months. From there I'd like to be able to install the most recent ubuntu version.

I think it's just a matter of configuring ubiquity, but don't know if this is the case and how exactly do this.

EDIT: Let's clarify the persistent thing:

• suppose I have my USB with ubuntu precise on it
• suppose quantal is out in the wild
• suppose that I want to install quantal on the hdd of a computer
• suppose that I want/can use only the USB drive with precise on it
• I should erase/upgrade precise on the USB drive to quantal and then install it on the hard drive

I don't want to touch my ubuntu install on the USB drive, I'd like to be able to install the newer ubuntu version (quantal) on a hdd from another one (precise) on a USB drive

I'd better avoid upgrading the installation on the hdd

Hope this helps