Test your bootable USB drive with QEMU

Step 0
Find the Bus and Device ID:

$ lsusb
...
Bus 001 Device 008: ID 0781:5151 SanDisk Corp. Cruzer Micro Flash Drive
...

Step 1
Boot with QEMU:

sudo qemu-system-x86_64 -m 512 -enable-kvm -usb -device usb-host,hostbus=1,hostaddr=8

You need to run this with root privilegues, because QEMU needs access to the corresponding files under /dev/bus/usb/.