Boot sector

A boot sector is the sector of a persistent data storage device (e.g., hard disk, floppy disk, optical disc, etc.) which contains machine code to be loaded into random-access memory (RAM) and then executed by a computer system’s built-in firmware (e.g., the BIOS).

— Wikipedia

Usualy it’s very first sector of the disk.

Unified Extensible Firmware Interface (UEFI)

Does not rely on boot sectors, instead it uses bootloaders (EFI application file from EFI partition or even USB device).

UEFI support code signing, so it’s possible to verify that bootloader is not modified.

Partition tables

Usually disks must be partitioned and there are two definitions how to store partition information.

1. MBR (Master Boot Record) - is the first sector of a data storage device that has been partitioned. The MBR sector may contain code to locate the active partition and invoke its volume boot record.
2. VBR (Volume Boot Record) - is the first sector of a data storage device that has not been partitioned, or the first sector of an individual partition on a data storage device that has been partitioned. It may contain code to load an operating system (or other standalone program) installed on that device or within that partition.

The maximum capacity of MBR partition tables is only about 2 terabytes.

MBR partition tables can have a maximum of 4 separate partitions, but one of those partitions can be configured to be an extended partition, which is a partition that can be split up into an 23 additional partitions. In total MBR can have 23 + 3 = 26 partitions.

To boot from an MBR disk, the boot mode should be Legacy BIOS instead of UEFI and the boot mode is UEFI.

GUID Partition Table (GPT) is a standard for the layout of partition tables of a physical computer storage device, such as a hard disk drive or solid-state drive, using universally unique identifiers, which are also known as globally unique identifiers (GUIDs). Forming a part of the Unified Extensible Firmware Interface (UEFI) standard Unified EFI Forum-proposed replacement for the PC BIOS), it is nevertheless also used for some BIOSs, because of the limitations of master boot record (MBR) partition tables, which use 32 bits for logical block addressing (LBA) of traditional 512-byte disk sectors.

— Wikipedia

GPT partition tables offer a maximum capacity of 9.7 zetabytes. 1 zetabyte is about 1 billion terabytes.

GPT partition tables allow for up to 128 separate partitions.

I use GUIDs in my fstab, so I can easily identify partitions.