Actually there are some details involved with the process. To boot-up a device, it must be formated with a partition that begins with a specific code on the first sectors, these partition area are called 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.