> [ .. ] Inserting an USB drive before boot breaks booting.

Only if the machine's BIOS is configured to give bootable USB devices boot-order priority. So it's not about Linux -- in fact, the same thing would happen on a Windows machine.

Remember that in a properly configured Linux install, the boot partition is identified by UUID, not hardware identifier (in /etc/fstab). Consequently if you change a drive's hardware connection point, the system still boots.