GPT and MBR, what’s the difference ?

If you have dabbled with your hard disk and is always doing formatting and partitioning, you will surely come across the term “MBR” and “GPT”. You probably are wondering, what are the differences between MBR and GPT and is there any benefit using one over the other? We wil clear your doubt in this article. GPT is the new 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 — Mac OS X, Linux, and other operating systems can also use GPT.

Hard Disk Partitions

You probably know that you can split your hard disk into several partitions. The question is, how does the OS know the partition structure of the hard disk? That information has to come from some where.

This is where MBR (Master Boot Record) and GPT (Guid Partition Table) come into play. While both are architecturally different, both play the same role in governing and provide information for the partitions in the hard disk.

Master Boot Record (MBR)

MBR is the old standard for managing the partition in the hard disk, and it is still being used extensively by many people. The MBR resides at the very beginning of the hard disk and it holds the information on how the logical partitions are organized in the storage device. In addition, the MBR also contains executable code that can scan the partitions for the active OS and load up the boot up code/procedure for the OS.

For a MBR disk, you can only have four primary partitions. To create more partitions, you can set the fourth partition as the extended partition and you will be able to create more sub-partitions (or logical drives) within it. As MBR uses 32-bit to record the partition, each partition can only go up to a maximum of 2TB in size. This is how a typical MBR disk layout looks like:

There are several pitfalls with MBR. First of all, you can only have 4 partitions in the hard disk and each partition is limited to only 2TB in size. This is not going to work well with hard disk of big storage space, say 100TB. Secondly, the MBR is the only place that holds the partition information. If it ever get corrupted (and yes, it can get corrupted very easily), the entire hard disk is unreadable.

MBR’s Limitations

MBR standards for Master Boot Record. It was 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 boot. If you have Linux installed, the GRUB boot loader will typically be located in the MBR.

MBR works with disks up to 2 TB in size, but it can’t handle disks with more than 2 TB of space. 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.

MBR became the industry standard everyone used for partitioning and booting from disks. Developers have been piling on hacks like extended partitions ever since.

GUID Partition Table (GPT)

GPT is the latest standard for laying out the partitions of a hard disk. It makes use of globally unique identifiers (GUID) to define the partition and it is part of the UEFI standard. This means that on a UEFI-based system (which is required for Windows 8 Secure Boot feature), it is a must to use GPT. With GPT, you can create theoretically unlimited partitions on the hard disk, even though it is generally restricted to 128 partitions by most OSes. Unlike MBR that limits each partition to only 2TB in size, each partition in GPT can hold up to 2^64 blocks in length (as it is using 64-bit), which is equivalent to 9.44ZB for a 512-byte block (1 ZB is 1 billion terabytes). In Microsoft Windows, that size is limited to 256TB.

From the GPT Table Scheme diagram above, you can see that there is a primary GPT at the beginning of the hard disk and a secondary GPT at the end. This is what makes GPT more useful than MBR. GPT stores a backup header and partition table at the end of the disk so it can be recovered if the primary tables are corrupted. It also carry out CRC32 checksums to detect errors and corruption of the header and partition table.

You can also see that there is a protective MBR at the first sector of the hard disk. Such hybrid setup is to allow a BIOS-based system to boot from a GPT disk using a boot loader stored in the protective MBR’s code area. In addition, it protects the GPT disk from damage by GPT-unaware disk utilties.

GPT’s Advantages

GPT stands for GUID Partition Table. It’s a new standard that’s gradually replacing MBR. It’s associated with UEFI — UEFI replaces the clunky old BIOS with something more modern, and GPT 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.

This system doesn’t have MBR’s limits. Drives can be much, much larger and size limits will depend on the operating system and its file systems. GPT allows for a nearly unlimited amount of partitions, and 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.

On an MBR disk, the partitioning and boot data is stored in one place. If this data is overwritten or corrupted, you’re in trouble. In contrast, GPT stores multiple copies of this data across the disk, so it’s much more robust and can recover if the data is correupted. GPT also stores cyclic redundancy check (CRC) values to check that its data is intact — if the data is corrupted, GPT can notice the problem and attempt to recover the damaged data from another location on the disk. MBR had no way of knowing if its data was corrupted — you’d only see there was a problem when the boot process failed or your drive’s partitions vanished.

Compatibility

GPT drives tend to include a “protective MBR.” This type of MBR says that the GPT drive has a single partition that extends across the entire drive. If you try to manage a GPT disk with an old tool that can only read MBRs, it will see a single partition that extends across the entire drive. The MBR ensures the old tools won’t mistake the GPT drive for an unpartitioned drive and overwrite its GPT data with a new MBR. In other words, the protective MBR protects the GPT data from being overwritten.

OS Support

Windows can only boot from GPT on UEFI-based computers running 64-bit versions of Windows 10, 8.1, 8, 7, Vista, and corresponding server versions. All versions of Windows 10, 8.1, 8, 7, and Vista can read GPT drives and use them for data — they just can’t boot from them without UEFI.

For Windows, only the 64-bit version of Windows from XP onward support booting from GPT disk. If you are getting a laptop pre-installed with 64-bit Windows 8, most probably it is using GPT. For Windows 7 and earlier version, the default configuration will be MBR instead of GPT.

Other modern operating systems can also use GPT. Linux has built-in support for GPT. Apple’s Intel Macs no longer use Apple’s APT (Apple Partition Table) scheme are using GPT by default and you won’t be able to install Mac OS X (without tweaks and hacks) on a MBR system. Mac OS X will run on MBR disk though, it is just that you won’t be able to install on it.

Most Linux kernels come with support for GPT. Unless you are compiling your own kernel and you didn’t add this feature in, you should have no problem getting your favorite distro to work in GPT disk. One thing to note, you wil have to use Grub 2 as the bootloader.

Conclusion

In most cases, you will be fine with either MBR or GPT. It is only in situation where you need to install Windows on a Mac, or when you need to have a partition bigger than 2TB, that you need to use GPT, or convert MBR to GPT. Also, for the newer model of computer that uses UEFI, it will only support GPT.