If you are hurrying to reply, System → Administration → StartUp Disk Creator — no, that's not what I'm talking about.
I want to try Ubuntu 11.04's Unity without touching my existing Ubuntu install.
To do this, I need to install the nVidia drivers first (sigh).
To do this, I need changes to persist a reboot.
To do this, I need to really install Ubuntu on a USB key.
How do you do that?
What I tried
I tried to make a USB key from Testdrive, then boot from it, then choose "Install Ubuntu." The installer refused to install to the installation media itself.
I tried, from my installed copy of Ubuntu:
sudo kvm /dev/sdb --cdrom .cache/testdrive/iso/ubuntu_natty-desktop-i386.iso
…but the installer didn't detect the disk properly.