A quick guide to make an USB Flash Drive bootable. This is usefull if you use Microsoft Windows installation images.
Open Command Prompt as an Administrator and go to town with these command steps:
diskpart
list disk (note the drive number of the USB flash drive)
select disk "drive number"
clean
create part pri - NOTE: if flash drive is larger than 32Gb use size=32000 or 16000 to limit size of partition for FAT32 file system
select part 1
format fs=fat32 quick
active
exit
Now the USB flash drive is ready for a Windows installation image
Source: Microsoft