Have reviewed the appropriate instructions but the installation still fails, as it appears the USB drive 'disconnects' and the boot process no longer sees it connected. I know the key works as I've been able to boot from it on a normal Windows machine. The 1st time through if fails trying to boot from the USB. The 2nd time through, after a reset, it doesn't even attempt to boot from the USB. This is on a brand new iMac 27 5K with El Capitan and Parallels 11.