http://kb.digium.com/articles/FAQ/How-to-run-Ultimate-Boot-CD-from-USB-stick-Linux

Mount the latest UBCD ISO or CD.

Extract UBCD files to a temporary location:

mkdir ~/Desktop/UBCDextracted

rsync -avr /media/<username>/UBCD528/* /home/<username>/Desktop/UBCDextracted/

sudo su

Determine which USB drive:

fdisk -l

Blank the USB drive (optional):

dd if=/dev/zero of=/dev/sdX

Create a FAT32 partition and filesystem:

Make the partition type 'c' (W95 FAT32 (LBA)) and 'active' (bootable).

fdisk /dev/sdX

mkfs.vfat -F 32 -n UBCD /dev/sdX1

Replug the USB flash drive to mount the filesystem.

cd /home/<username>/Desktop/UBCDextracted/

rsync -avr /home/<username>/Desktop/UBCDextracted/* /media/<username>/UBCD/

dd if=./ubcd/tools/linux/ubcd2usb/mbr.bin of=/dev/sdX

chmod +x ./ubcd/tools/linux/ubcd2usb/syslinux

./ubcd/tools/linux/ubcd2usb/syslinux --install --directory /boot/syslinux /dev/sdX1

sync