When you boot the computer from USB, and you see a message “EFI USB Has been blocked by the current security policy”, access to this USB device is blocked.
This message arises from the “Secure Boot” protection that is enabled on your computer, more specifically in the UEFI/BIOS.
Secure Boot is a security feature designed to ensure the integrity of a computer's boot process by ensuring that only trusted and signed software and firmware can run during boot. This feature is usually implemented in the Unified Extensible Firmware Interface (UEFI) firmware of modern computers and is intended to prevent malicious software or malware compromises a computer's boot process.
When Secure Boot is enabled in the BIOS or UEFI of a computer, the system checks whether the boot loader and operating system kernel have digital signatures from a trusted certificate authority. If the software is not signed or the signature cannot be verified, the boot sequence will be blocked and an error message will be displayed.
The message “EFI USB has been blocked by the current security policy” usually appears when Secure Boot is enabled and you try to boot the computer from an external EFI USB drive that is not signed or whose signature cannot be verified.
The solution is to temporarily disable Secure Boot in the UEFI/BIOS. This is how it works.
Disable “EFI USB has been blocked by the current security policy” notification
Make sure your computer is turned on and restart it if it isn't already.
During startup, you will need to access your computer's BIOS or UEFI screen. This is usually done by pressing a specific key, such as F2, F10, Delete or Esc, depending on the brand of computer. There will usually be a short message displayed on the screen during boot up to tell you which key to press to start it Enter BIOS or UEFI. Consult your computer's manual for specific instructions if you're unsure.
Once in the BIOS or UEFI, look for the option called “Secure Boot” or something similar. This option can usually be found under the “Security” or “Boot” tab in the BIOS or UEFI menu.
Select the “Secure Boot” option and change the setting from “Enabled” to “Disabled” or a similar option depending on the choices available in the BIOS or UEFI firmware.
Save the changes and exit the BIOS or UEFI menu. This is usually done by following the on-screen instructions. Usually you have to press a key (e.g. F10) to get the save changes and the BIOS or leave UEFI.
After you disable Secure Boot, restart your computer to make sure the changes take effect.
I hope to have informed you with this. Thank you for reading!