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.
You would need to get the flash drive formmatted as a floppy again (FDD) or try selecting that option in the BIOS. Note that most current computers don't use the "USB ###" type options (FDD, HDD, ZIP, etc.). Instead, they recongize the flash drive as a hard drive device. FSi8N2r.