HELP!! How to boot latitude e5500 from a USB-Device(for examp. usb floppy drive)???

i want to install a multi-boot manager on my latitude e5500, but it uses freedos and needs a floppy drive to install. so i want to buy external usb floppy drive. For testing i connected a usb-dvd drive to make a test boot from the usb-port. Although the BIOS in my Latitude e5500 has an option to let laptop boot from the "USB Storage Device", i suppose, it means a USB external device e.g. USB-stick, USB-HDD or USB-CDRom, USB-floppy-dirver etc. but it doesnt work. i have no idea what's wrong with it, or is it possible at all? if yes, how to make it work.

thanks in advance

UPTATE: I tried a Gparted-Live-USB memory device test and connected a USB external HDD, both were showed in Boot Menu(F12 by booting) als "USB Storage Device" and worked. But still didnt work with my USB-DVDRom.