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i've seen the definition that the mbr is on the first record of a disk and what it does, but what if you have two disks, like hda and hdb and they can both boot. is the mbr only on the primary master(hda) or do they both have individual boot records?
The HDD linked as master will hold the MBR.
If you dual boot, just remember, when installing different flavours of Windows.
Oldest first.
To be exact, the MBR is on first hard drive found by your bios. Usually, the first hard drive is found on the Master of IDE1 Channel so the MBR is there also.
Each (physical) disk formatted to be a boot device will have a (valid) MBR.
However, BIOS, due to its limitations, scan all device in the boot order (specified on boot parameters), seeking out for a good mbr. The first one found will be use as boot device.
On some bios will can open a boot menu (with F8) and choose from where you want to boot.
MBR, superblocks and partition table are part of disk formatting structure.
As an example, if you install 2 HDD on a machine and install Windows, Windows installer will install the O.S. on the 1st disk (by default you can choose). The 2nd disk will not contain a valid MBR.
to expand on your definition, a MBR is the Master Boot Record that tells the bios what to do next, the one that is used is always on the primary master, and pretty much tells the bios to go to the boot loader.
NOW for the twist. it is possible using lilo or grub to load the bootloader into the MBR area so that you can dual boot *nix and windows. and then lilo/grub will point to the kernel of linux or the ntldr for windows (windows's native bootloader)