I had similar USB issues as mentioned in the other threads [1] [2].
To work around them I used a USB 3.0 flash drive plugged into one of the front blue 3.0 ports, as jonathon.hall said above. Also, creating the OS install disk on another computer will ensure the image is not corrupted due to the USB signaling issues. Lastly, I found if you plug in your USB drives before attaching the power supply to the mini, then they will usually be detected and function correctly on the next boot.
I am not sure if this is a Coreboot issue or an issue with the hardware that is also present on the proprietary firmware for this board. But it is a big inconvenience not being able to use a lot of USB 2.0 devices. I have not heard of such issues with Thinkpads and other laptops running Coreboot/Libreboot, so it may just be the USB controllers on this board do not play well with Coreboot yet.