Optical drive (CD-ROM/DVD): Even when you don’t choose the optical drive as the primary boot device, many PCs automatically detect a bootable disc in the optical (CD or DVD) drive. When that happens, an option is displayed on the screen, something like

The boot order is a priority list. For example, if “USB drive” is above “hard drive” in your boot order, your computer will try the USB drive and, if it’s not connected or no operating system is present, it’ll then boot from the hard drive. To

Change the boot order in BIOS so the CD, DVD, or BD drive is listed first. Some computers are already configured this way but many are not. If the optical drive is not first in the boot order, your PC will start "normally" (i.e., it'll boot