Hi sorry danjac, so this works fine for yoga tab 3 pro. I have done this on lollipop and marshmallow without problems. I must stress that you try this at your own risk. Becareful in the bios as you don't want to do anything to create a nice paperweight.
Manual boot to bios,
Try this at your own risk!!!!
This is magic code from Microsoft apparently. Feel free to google it.
These commands need entering one by one from adb shell. They need to be exactly right!!!!!!!!!!!
mount -t efivarfs efivarfs /sys/firmware/efi/efivars
printf "\x07\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\ x00" > /sys/firmware/efi/efivars/OsIndications-8be4df61-93ca-11d2-aa0d-00e098032b8c
Tablet should boot to bios, you can navigate with the buttons. POWER=enter, vol+/- =navigate menus and long press vol+=Esc.
After using the commands from adb once the reboot to bios option should work in twrp.
Not at my tablet at the moment, will post which options need changing to allow usb keyboard and usb stick support shortly.
android. Uefi secure BOOT=disable.
South cluster configuration. Usb configuration/default drd CONFIG=hostmode
If you want to be able to interupt boot to select usb device also disable fast boot under boot catergory. Then use f7 to select boot device.
After booting another os from usb it often takes 3 boots to get back to android, don't panic.
Oh and lastly a powered usb hub is needed.
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