As it happens, the problem isn't with either BitLocker or with your USB drives. It's with your laptop's BIOS. Specifically, in some cases the BIOS software — the software that controls your computer until the operating system loads — doesn't recognize USB drives over a specified capacity. Once the computer's operating system kicks in, you won't have any problem using that larger capacity USB drive. But if you're trying to boot from it, the BIOS simply won't see it. The quick solution? Try a USB drive with a capacity of 512 megabytes or less.
Posted by rlssec