fast Linux distro only to let boot from USB

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http://unix.stackexchange.com – Older pc's BIOS don't have ability to boot from usb and no upgrade from the mb manufacturer that makes that possible. Usualy i load unetbootin to make usb bootable. since this fails here, i want to ask for for a fast linux which starts from a 32mb harddrive and than enables the usb to boot from. i already tried grub but as far as i know u have to predeclare the systems used, right? since i frequently switch from tinycore, arch and other lean distro's i want to be flexible. Thank you (HowTos)