Then, there is the actual bootloader itself, i.e. the first bit of code that executes when a computer boots from USB. Unfortunately, HDD/USB and ISO bootloaders are very different beasts, and the BIOS or UEFI firmware also treat USB and optical media very differently during boot. What is the difference between USB CD and USB FDD? Only the option that includes the word “USB” is a USB option. FDD is a floppy disc drive, CD ROM is a CD drive, and HDD is a hard drive. How do I boot from USB FDD? Go into the BIOS, and go to the page that determines the boot order. gjX9.