A partition structure defines how information is structured on the partition, where partitions begin and end, and also the code that is used during startup if a partition is bootable. If you’ve ever partitioned and formatted a disk—or set up a Mac to dual boot Windows—you’ve likely run into the two main partitioning structures: Master Boot Record (MBR) and GUID Partition Table (GPT). GPT is a newer standard and is gradually replacing MBR. GPT brings with it many advantages, but MBR is still the most compatible and is still necessary in some cases. This isn’t a Windows-only standard, by the way—Mac OS X, Linux, and other operating systems can also use GPT.
MBR was first introduced with IBM PC DOS 2.0 in 1983. It’s called Master Boot Record because the MBR is a special boot sector located at the beginning of a drive. This sector contains a boot loader for the installed operating system and information about the drive’s logical partitions. The boot loader is a small bit of code that generally loads the larger boot loader from another partition on a drive. If you have Windows installed, the initial bits of the Windows boot loader reside here—that’s why you may have to repair your MBR if it’s overwritten and Windows won’t start. If you have Linux installed, the GRUB boot loader will typically be located in the MBR.
MBR does have its limitations. For starters, MBR only works with disks up to 2 TB in size. MBR also only supports up to four primary partitions—if you want more, you have to make one of your primary partitions an “extended partition” and create logical partitions inside it. This is a silly little hack and shouldn’t be necessary.
GPT is a newer standard that’s gradually replacing MBR. It’s associated with UEFI, which replaces the clunky old BIOS with something more modern. GPT, in turn, replaces the clunky old MBR partitioning system with something more modern. It’s called GUID Partition Table because every partition on your drive has a “globally unique identifier,” or GUID—a random string so long that every GPT partition on earth likely has its own unique identifier.
GPT doesn’t suffer from MBR’s limits. GPT-based drives can be much larger, with size limits dependent on the operating system and its file systems. GPT also allows for a nearly unlimited number of partitions. Again, the limit here will be your operating system—Windows allows up to 128 partitions on a GPT drive, and you don’t have to create an extended partition to make them work.
Master Boot Record (MBR) –> 2TB (older os) (32bit) physical —> Virtual
GUID Partition Table (GPT) –> 16exaByte (64bit) (Repair) physical -(need tools)–> Virtual