GPT vs. MBR Booting

Most of the time, we let our computers’ boot just happen, but sometimes we need to control it. One of those times is when you want to dual boot. The way your disk is organized affects what you need to do and think about. The way computers boot and have been booting is by using the Master Boot Record. That was the old way, but you will still see partitioning software give you the option to use this system. GPT means GUID Partition Table; it was introduced to address BIOS limitations, one being the size of disk it can address. To use GPT, you must have a UEFI based computer. In 2021, you do! Just watch out for decades-old hardware if you are a tinkerer. Note that you can still keep using MBR if you wish to do so.

The standards in your start-up.

Let’s make sure we know which standard does what:

BIOS checks your hardware before it looks for the disk and the MBR. The MBR is a section of the disk at the physical beginning. This space is only at that beginning. So BIOS looks for the MBR, which in turns points to the Operating System.

UEFI does the same job as the BIOS, but instead of pointing to a specific address on the disk, it searches for your ESP. The ESP is the partition where you have all the files that run your boot manager. You can point to any *.efi file; these files are executable and most commonly run grub.

The interesting part is that UEFI can also point to your MBR partitioned disk. This was necessary since many systems only had those disks and needed to stick with them for a few generations. This means that you can still choose to partition your disk using MBR. You will also have no problems doing this unless your disk is over 2.2 terabytes.

Using GPT on your disk has many advantages, though, and the added complexity is very small. A final detail that you can add to your disk is the PMBR. The PMBR will act as the MBR when the hardware cannot handle it. It is only a backward compatibility issue.

How do I use this?

This is interesting for you to know when you install a new distribution. Most distributions have built-in partitioning, but some do not. When you have finished the installing process, you may still need to partition new disks; hence you should know the difference between the partitioning standards. If you have no particular demands, you should use GPT and any standard that the distribution suggests.

Reasons to choose GPT over MBR

This is the simplest way to partition your drive, don’t make that your reason for doing so! Even compatibility is usually not a reason since your partitioning software will create the PMBR mentioned earlier. You would be forced to have, at least, PMBR on any USB drive that you plan to use on really old hardware. Any hard disk that you install in a machine with UEFI, you should use GPT. The reasons are many. Your disk’s size is not your main concern; in this case, instead, you have many features that speak for GPT.

One feature is that you can have as many partitions as your OS allows. The initial limitation is usually 128 partitions, but the standard allows many more. If you need more partitions, you probably have chosen the wrong strategy and should think again. The second feature you should appreciate is that the table is in two places on the disk. On an MBR disk, you have the table on the first sector and nowhere else! Using GPT, you have the table in two places; the beginning and end of the disk. On top of that, it is really simple to make a backup copy of the ESP to external media. GPT also uses CRC to check that the partition table is healthy. This can give you ample warning that one of the copies is corrupted. In this case, the system uses the second copy and boots as usual. If this is your situation, start gdisk ‘/dev/sdX’, type ‘v’ to verify your disk, and then ‘w’. You will end up with both tables in a good state. WARNING: If you have physical problems with the disk, you may end up with an un-bootable disk. Keep Backups!

Moving from MBR to GPT

Since you most likely want to use GPT, there is a way to move to MBR. You can usually achieve this without re-writing the entire disk, though you should keep backups!

The earlier mentioned ‘gdisk’ utility can do it for you. It is even simpler to use ‘cgdisk’, where you have a list of partitions listed and options at the bottom. It looks the same as ‘cfdisk’ and works almost the same. When you start ‘cgdisk’, you get warnings that the disk is an MBR disk and that ‘gdisk’ will convert your disk. This happens in memory, and you can back out at any time. When you have verified that the changes are good, cross your fingers and write to disk. If you have a decent and healthy disk, you should end up with a GPT disk. This can fail since some programs that create MBR disks do not align correctly, and ‘gdisk’ will not recover your disk.

Conclusion

In your current system, using MBR is usually unnecessary. If you have very old hardware, you may have some use of it, but during the operation of hardware newer than 2007, you are close to guaranteed to have support for GPT. With GPT being more robust and secure, you should use GPT except in extremely rare cases. Have fun with your portable media, and if you can still keep a BIOS machine running; Kudos! It is an achievement in itself!