Linux is broken from this point of view. 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.
Fixed that for you. It used to be normal to use the device path (/dev/hd* or /dev/sd*) to reference the filesystem partitions. Using the UUID or the by-id symlink instead is a novelty, introduced precisely to fix these device enumeration order issues.
Two other people were able to concisely explain the problem instead of being rude and condescending.