Na verdade, há alguns detalhes envolvidos no processo. Para inicializar um dispositivo, ele deve ser formatado com uma partição que começa com um código específico nos primeiros setores. Essas áreas de partição são chamadas MBR .
A Master Boot Record (MBR) is the bootsector of a hard disk. That is, it is what the BIOS loads and runs, when it boots a hard disk. The MBR is the very first sector of the hard disk; it contains an MBR Bootstrap program (described below), and a Partition Table. Devices that emulate a hard disk during system initialization must also contain an MBR, because they must also have Partition Tables -- even if they are not bootable. The BIOS will only boot an MBR from a device if that device is in the "boot sequence" stored in CMOS, and if the MBR on the device is formatted correctly. On the other hand, if a device is not in the boot sequence (but has a "drive number"), it is still possible for a Real Mode program (such as another MBR or bootloader) to load and boot that device's MBR directly.