Thanks for the A2A.
You are mixing terms. MBR and GPT are partition schemes an GRUB (GRand Unified Bootloader) is a bootloader.
A bootloader is more or less an application that LOADS something upon BOOT. Usually something like an operating system, a live system, a rescue disc or an installer on an install medium. Grub is the default bootloader for many Linux distributions on desktop and servers and has been for many years. Although meanwhile there are other options, it is still primarily and most often being used with Linux. Also, Grub can be used with both, MBR AND GPT partition scheme.
MBR and GPT refer to setups on a hard disk.
GPT stands for GUID partition table and GUID stands for Global Unique IDentifier. Meaning, that in the header of the hard disk a unique identifier is being stored. This for example is being used to activate Windows 10.
MBR is actually an acronym for Master Boot Record. This basically refers to the beginning of the hard disk. In this particular context it refers to an old partition scheme and the master boot record is where the bootloader would sit in this case. Also, the MBR partition scheme doesn’t include a unique identifier in the header.
Unlike with GPT, which is a newer partition scheme that is usually being used with UEFI BIOS. In this case an EFI shell searches for a .efi file on one of the partitions on your hard drive, which then boots the operating system.
In conclusions, with MBR partition scheme the bootloader (in your example Grub) would be stored in the master boot record in the header of the hard drive, BEFORE the partition table and OUTSIDE of all partitions. In case of GPT partition scheme a .efi file would be used by an EFI shell to start the operating system. This .efi file would be located AFTER the partition table on the hard drive and ON a (FAT32) partition.
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