[ Thanks to An Anonymous Reader for
this link. ]
“The MBR partitioning system is a hodge-podge of data
structure patches applied to overcome earlier limits. The MBR
itself resides entirely on the first sector (512 bytes) of a hard
disk. The first 440 bytes of the MBR are devoted to code: the boot
loader. The BIOS reads this code and executes it when the computer
boots.“Following the code area, the MBR stores data on four
partitions, known as primary partitions. Each partition is
described in two ways: using cylinder/head/sector (CHS) notation
and using logical block addressing (LBA) notation. The CHS notation
is almost a historical footnote today, because it’s a 24-bit
number. This means that it’s limited to describing 8GB-disk areas.
The 32-bit LBA values permit 2TB sizes. This 2TB ceiling is not
easily overcome; there simply aren’t any unallocated fields left in
the MBR that could be used to add more bits to the LBA
addresses.“In addition to the looming 2TB problem, the MBR presents other
difficulties. Chief among these is the limitation of four primary
partitions. To work around this limitation, it’s possible to set
aside one primary partition as a placeholder (known as an extended
partition) to hold an arbitrary number of additional partitions,
known as logical partitions. This is, however, an ugly workaround
that creates its own problems, such as difficulties installing
multiple operating systems when too many of them want too many
primary partitions to themselves.”