I'm trying to boot a usb system image, but I can't find the option in my BIOS?

I have a Lenovo Yoga 3 pro with win8.1 preinstalled. I wasn't able to use the built in image creator to install the image directly onto the usb, so I mad a partition on an external HDD, and then used minitool (a partition manager) to copy that partition onto my flash drive. I have booted from USB before, I know that my system is capable of doing that, but the flash drive doesn't appear in my bios menu. Even worse, I can't see the flash drive in windows explorer anymore. My end goal is to use a different OS, but to keep a copy of my preinstalled OS and partitions that I could restore whenever I wanted. This is causing me a lot of headaches… Any ideas?

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