Hi there, this is Marc. Thanks for reaching out to the community.
Windows cannot set an external USB drive as the primary system drive. To use it as your main drive, you would need to install Windows directly onto it, which is only supported with Windows To Go on certain enterprise editions and certified drives. A better option is to replace the internal drive with a larger SSD or HDD and reinstall Windows, or clone your existing system onto the new internal drive. You can still use the external drive for storage, but it cannot act as the primary boot drive in standard Windows editions.
Best regards,