There should be a hint for adding the "bootable" flag for /dev/sda. Without this some (or all) systems won't boot.
Did you miss this instruction at the beginning? "Create a FAT32 partition on your USB flash drive and mark it bootable using fdisk. An example partitioning scheme can be seen below:" # fdisk -l /dev/sda (...) Device Boot Start End Blocks Id System /dev/sda1 * 1 1017 1023580 b W95 FAT32