After days of trying to help a friend reset forgotten passwords on two Surface Pro 4's, and trying and failing at many suggestions found on the web, here is what worked for me on both machines. My friend had purchased a copy of Windows 10 on USB but both
devices refused to boot from the new USB stick with windows on it. Even after I went into the UEFI and set USB as the only bootable option, I also selected "None" in the Security setting (You will need to selected these settings for this to work). I also created
a bootable USB stick using my Windows 10 desktop, but both surfaces failed to boot from that as well even after repeated attempts and UEFI settings. Ok here is what did work. After many in this thread mention successful boots after switching USB sticks I decided
to try my own fix. I went to Staples and purchased a 32 GB Lexar S75 USB 3.0 Jumpdrive. It was on sale for $16.99 plus $1.51 tax (total $18.50). I then formatted the drive in my Windows 10 desktop (select fat32). I then copy all of the files from the new Windows
10 USB stick that would not boot on to the formatted Lexar 3.0 USB stick. I then put the Lexar USB stick into one of the Surface Pro 4's and pressed the start button. Success.... it booted from the Lexar 3.0 USB stick and showed me the options for installing
or repairing windows on the Surface Pro 4.
** Remember before booting from the USB stick, you must go into the UEFI by holding down the shift key on your surface keyboard, left click on the power button on the bottom right of the lock screen, and select reset (the surface keyboard works best since
you will need the USB port for your USB Stick) when the surface restarts you will get a blue screen with several options. On the blue screen select "Troubleshoot", then Select "Advance options", then select "UEFI Firmware Settings", then hit "Restart". Once
the machine restarts you will be on a screen saying "Surface UEFI". Go to security, and in "Change configuration", select "None". Also turn off TMP. Then go down to "Boot configuration" and make sure only "USB Storage" is selected (black square check box).
Also on that screen, go to Advance options and make sure only "Enable from USB devices" is turned on. If this is done correctly you will get a red bar with a white lock on the top portion of your screen during the bootup procedure. Now hit exit and restart.
Make sure the Lexar USB is inserted in the machine.
This is the process that worked for me, and I just wanted to share all of the steps that I followed. I think it may also work with just a bootable USB that you can create from any windows computer since my problem was the machine failing to boot from any
USB stick during the bootup. procedure. The problem may be isolated to the USB stick being used. I suggest using nothing less than a 3.0 USB stick when trying to boot from USB.
Good Luck.