Just found this..
"When I installed Windows on my PC (and this may or may not be the right way to do it, but it worked for me), i had the same problem. I was able to make it work but taking out the USB as soon as the PC was powered down in the reboot phase of the install. My best guess is that your motherboard doesn't know if you have Windows installed or not, and just sees that in Boot priority 1 is USB, and there's a .iso on the usb, so run it, and then there goes your old (partial) install. I would restart the entire process, wipe the SSD, etc, and upon reboot take out he USB before the computer has a chance to turn on again and the motherboard tries to boot off the USB."
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