I ran into this same problem and I fixed it by using parted to change the filesystem label from msdos to gpt.
Open your USB flash drive with parted:
sudo parted /dev/sdX
print command to see what the current filesystem label is. This is what I get:
GNU Parted 3.2
Welcome to GNU Parted! Type 'help' to view a list of commands.
Model: SanDisk U3 Cruzer Micro (scsi)
Disk /dev/sdh: 2048MB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Number Start End Size Type File system Flags
1 1049kB 2048MB 2047MB primary fat32 boot
Notice the partition table is MSDOS. When you use dd to write the ISO to your flash drive, it does not appear to write a partition table with it so you are stuck with whatever format it had before. Anyway, use the
mklabel command to change the label from msdos to gpt.
(parted) mklabel gpt
Warning: The existing disk label on /dev/sdh will be destroyed and all data on this disk will be lost. Do you want to continue?
Now use dd to write the ISO to your flash drive and it should boot. At least this worked for me, YMMV.