Old thread I know.

Here's how I solved my post USB boot "_" error on my Lenovo S20

Firstly I have a new case, new GPU, new ram, new SATA ssd. So every time I start my pc that temperature error pops out at the loading screen saying a sensor is missing, you have to spam F2 for it to boot normally or else it enters a loop sequence saying that no bootable drives are found.

After that keep in mind when you install windows everything the setup restarts you have to press F2 or else the setup process quits and you have to do every thing again.

Secondly disable fast boot in the bios, and when you create a bootable USB stick make it MBR partition(I used Rufus) because other partition schematics won't work in my case.

Plug in mouse, keyboard, bootable USB stick, press f1 to enter bios and make your USB stick your 1st boot able option.

Install windows on a empty ssd(use new command to format it, windows also creates a reserved partition 550mb usually ) and don't forget to press F2 when it restarts(2-3 times).

Cheers.