I want to install Debian directly onto my USB drive. Is there an easy way to do this directly without having to reboot to run the installer?
This doesn’t really install it, though, you can’t update or permanently edit and config, set up users, or anything like that. I would guess OP wants something more like booting the ISO in a VM, allocating a thumb drive to that VM, and then installing a full system to it with a boot loader.