Slax

“Not to mention that the installation itself is almost laughably
easy. You download a tarball, extract it to your USB stick and then
just run an included shell script to overwrite the devices MBR. It
literally takes 5 minutes, including the download time (well,
depends on your bandwidth, but you know what I mean).

“The USB version of Slax is persistent – it will actually
write changes to the disk, and allow you to set up your environment
the way you like it and keep it that way. If you mess up, you can
even reset it to a default state at boot time. It boots around 10
times faster than a Live CD and is quite responsive.”

Complete
Story