Knopperdisk is an operating system specifically designed for a USB pen drive and floppy disks. Knopperdisk is a fairly minimal, text system that can be used as a rescue disk, although the USB pen drive version has a few more software packages installed which are used for different purposes.
Despite the similarity in name to Knoppix, it is based on Gentoo Linux.
References
Knopperdisk Wikipedia(Text) CC BY-SA