There is no easy answer to that.
Partly because I think we are circling - an xyproblem - the underlying issue is the lack of understanding of the difference between BIOS/MBR and EFI/GPT.
Depending on the type of USB device it may report itself as REMOVABLE and installing Manjaro to a removable device must be done manually and special caution is needed both with the fstab generation and the boot loader installation.
There is several topics on how to target a removable device for installation. Just search the forum - one of them is this
Please familiarize yourself with the subtle differences as I am quite sure this is where the real issue is.