I have a Lenovo X220 that won't boot from USB Flash Drive.
I had tried multiple USB sticks, prepared as bootable in multiple ways. They boot on other PCs, including a T400. But they won't even show on F12 boot list on the X220. I also tried all the USB ports (so it is not a USB 3.0 issue).
The BIOS settings on the X220 is for Legacy and UEFI boot, Legacy first (HDD OS is legacy boot). UEFI USB and USB access to port is enabled. BIOS is the latest version.
Before proceeding, ensure that
The USB driver is up to date and usb ports work within your operating system. The Lenovo Recovery media from Advanced mode of Lenovo Rescue & Recovery tool is made. You'll get bootable stick with WinPE environment. Test the boot process. Or you can make Recovery stick from OS. You can also make the bootable media in Lenovo Factory Disks application. Test with that.
Check the boot exclusion list in BIOS as the USB flash drive could be there by default.
With a bootable USB Flash Drive plugged in, go into BIOS settings, Startup, select the >Boot header and press enter so you get the boot device order list. Scroll down past the last entry in this list, and you will jump to a boot exclusion list. Find your USB flash drive here and type "!" to get it off the exclusion list and on to the boot device order list.
Save BIOS changes. Now it will also show on F12 at boot, and boot.