The BIOS rules for adding or removing a bootable device are listed below:
If CMOS checksum occurs or the user loads CMOS setup defaults, a default boot priority list will be loaded. The default priority list is based on the order each boot device is found.
Any new bootable device added to the system will be placed at the end of the boot list.
Any device removed from the system will result in subsequent devices below the priority moving up in the priority list. In essence, the priority list will collapse without affecting the order off the remaining devices.
BIOS will notify the user if any new device is removed or added from the boot list.