Relationship between MBR / GPT / UEFI and their limitations

Question

I have a situations where i have to decide to use MBR or GTP and UEFI. I've read too many articles on internet, but I'm confused about some things :

1. I've read that MBR does not support drives with more than 2.2TB. But I have an external hard drive, 3TB, that uses MBR and I can use all its space. So is or not true that MBR support only 2.2 TB ? Screenshot for my External 3TB drive , made from Paragon Partition Manager

2. On all articles about GPT, always it's a relation with UEFI. But on my old PC, non-UEFI, I use a partiton tool that let me convert my disk to GPT. So GPT can be used only with UEFI, or can I create and use a GPT disk even on old PC without UEFI?

Answer 1

For your first question: MBR is limited to 2.2 TB. It's highly likely you think you are using all 3 TB but you are not, OR you think it's MBR when it's actually a GPT disk. I would check Disk Management which will tell the full story.

For your second question: hard drive partitions have nothing to do with BIOS/UEFI and more to do with the operating system. You can use a GPT partitioned drive in a BIOS (non-UEFI system) as long as the operating system supports it, however a UEFI system must use GPT.

Answer 2

Both of the previous answers (by Hennes and Moses) get some parts right and some parts wrong. Here's the correct set of answers:

1. To elaborate on what Hennes wrote, MBR is limited to partitions of 2^32 - 1 (that is, 4,294,967,295) sectors that start no later than the same value (counting starting from 0). Given a sector size of 512 bytes, this works out to a 2 TiB (approximately 2.2 TB) limit on partition size and start point. Technically, it's possible to stretch this to a disk size of just under 4 TiB by splitting the disk into two or more partitions, but that's unwise because many OSes, and probably many disk utilities, will flake out on such a disk. Thus, on a disk with 512-byte sectors, the safe limit for disk size under MBR is 2 TiB. Your external disk, however, probably uses 4096-byte sectors, which raises this limit to 16 TiB, which of course is much larger than the 3 TB disk size. Although there's a slim chance that Moses is correct and the disk is only being partially used, that's unlikely; 4096-byte sector sizes are very common on external disks and USB disk enclosures sold without disks installed, particularly when the disks are bigger than 2 TiB. To be sure, check the size of sectors on your disk. Most disk partitioning utilities provide a way to get at this information, but details vary greatly from one tool to another.
2. As Moses says, support for GPT vs. MBR is mostly an issue of OS support, not firmware (EFI vs. BIOS) support. That said, GPT was created as part of the EFI specification, so GPT is often associated with EFI. An old-style BIOS does not itself parse the partition table during the boot process; it just loads the first sector of the disk and executes whatever code it finds there. It's this boot loader, not the BIOS per se, that reads the partition table. OSes like Linux and FreeBSD, which use boot loaders that can understand GPT, can therefore boot from GPT disks even on old BIOS-based computers. It's usually possible to boot from an MBR disk in EFI mode, too, although this configuration is difficult to set up because most OSes' installers discourage such a configuration. All that said, there are occasional issues with firmware compatibility, such as firmware that sees a GPT disk and uses that as a cue to disable its BIOS compatibility support, thus blocking the GPT-on-BIOS option from working. Such issues are rare, even on older computers, contrary to what Hennes wrote. In fact, in my experience, these problems are more common on newer UEFI-based computers; their firmware, unlike a BIOS, parses the partition table, and so can use that information to disable BIOS/CSM/legacy support.

Both of these issues are covered in more detail in my GPT fdisk documentation. See in particular the sections entitled Working Around MBR's Limitations, Legacy BIOS Issues with GPT, and Booting from GPT.

Answer 3

I've read that MBR does not support drives with more than 2.2TB.

MBR has a limit in a field where the maximum number that that field supports times the old 512 bytes sector size was 2TiB. That limited it to disks of 2TiB (marketing speach 2.2TB) or less.

These days we also have consumer drivers with 4K sectors and the limit for MBR with those drives is eight times higher.

On all articles about GPT, always it's a relation with UEFI.

Not always. Just mostly.

An old motherboard with BIOS firmware usually does not understand GPT. That means it will either fail to grok the drive and not boot. Or when horrible implemented it may even fail to POST.

A motherboard with a moderm BIOS can use GPT drives just fine.

A motherboard with EFI firmware has to support GTP. Given the timeframe that GPT and EFI were introduced you usually see the first and the last. But that does not exclude the middle answer (A good BIOS working just fine with GPT).