If you take your existing Windows 7 or Windows 8 image installed in SATA hard disk, the changes that it will boot from a USB using a USB to SATA converter is very low.
You need to make some changes to your image in order to make your disk drive bootable in USB.
Please refer to the link below for the steps:
http://www.webpixel.gr/blog/boot-windows-7-from-usb-hard-disk
