1. Insert a USB key in the system.
2. Open a command prompt.
3. Use diskpart.exe to format the USB key as a single FAT32 partition spanning the entire key, and set the partition to active, as shown in this example.
   • diskpart
   • list disk (note what disk the USB drive is. In this example it is Disk 2…I)
   • select disk 2
   • clean
   • create partition primary
   • select partition 1
   • active
   • format fs=fat32 quick
   • assign
   • exit
   • Copy files from ISO to the USB key.

For NTFS bootable USB drives there are some additional steps.

1. Follow the steps above, but when formatting use format fs=ntfs quick.
2. Remount the USB key if diskpart ejects it.
3. Insert a Windows Operating System installer ISO image.
4. Open a command prompt as Administrator and navigate to the boot folder (mine ISO is mounted in J:) J:\boot>

5. bootsect /nt60 G: (where G: is the drive letter of your USB key). You should see something like this:
   J:\boot>bootsect.exe /nt60 G:
   Target volumes will be updated with BOOTMGR compatible bootcode.

   G: (\\?\Volume{db89f4a5-faa0-11e4-825a-989096b10a0c})

   Successfully updated NTFS filesystem bootcode.

   Bootcode was successfully updated on all targeted volumes.