I enjoy using Knoppix 6.2.1 on a 2Gb USB SD Card.
This is my way of 'dual-booting' my Dell 1545 Window 7 machine.

I keep copies of knoppix-data.img as a 'back-ups' on another
USB card to use if required to restore my working 6.2.1 setup.

I make additional copies from time to time, since I'm always
tinkering with Knoppix. These copies allow me to revert back to
earlier configurations, if necessary, while retaining whatever
changes that continue to serve my purposes.