Partitioning and Boot Structures

Dividing the Drive and Starting the OS

Learn how storage space is logically divided using partitioning standards like MBR and GPT, and understand the role these partitions play in the operating system boot process.
Author

Chuck Nelson

Published

October 22, 2025

1 Purpose

Once a drive is initialized, it is still a single, vast, empty space. This document explains the crucial next step: partitioning. Partitioning divides the raw disk into one or more logical sections that an operating system can manage. We will explore the two major partitioning standards—MBR and GPT—and see how they are fundamentally tied to the way a computer boots up.

2 What You’ll Learn

By the end of this reading, you will be able to:

  • Define the purpose of partitioning a storage drive.
  • Compare and contrast the MBR and GPT partitioning standards, including their limitations and benefits.
  • Describe the difference between primary, extended, and logical partitions in an MBR scheme.
  • Explain the role of the MBR boot sector and the EFI System Partition (ESP) in the boot process.

This reading maps to the following program and course learning outcomes:

  • Program Learning Outcomes (PLOs):
    • 3. Apply terminology and numeric or system concepts: This document defines the precise terminology for partition types (MBR, GPT, Primary, Logical, ESP) and their structural rules.
  • Course Learning Outcomes (CLOs):
    • 4. Configure hardware and basic network components: Partitioning a disk is a fundamental configuration task when setting up a new computer or drive.

This exercise develops the following skills, which align with the O*NET SOC Code 15-1232.00 for Computer User Support Specialists.

Learning Objective O*NET KSAs Technologies Used
Differentiate between MBR and GPT. Knowledge: Computers & Electronics
Abilities: Information Ordering, Category Flexibility		 MBR, GPT, UEFI, BIOS
Explain the role of partitions in booting. Knowledge: Computers & Electronics
Skills: Reading Comprehension		 Boot Sector, ESP

3 What is Partitioning?

Partitioning is the process of dividing a single physical drive into one or more regions, called partitions. The operating system treats each partition as a separate logical drive. This process creates a partition table on the drive, which is like a table of contents that tells the OS where each partition begins and ends.

There are two main standards for how this partition table is created and managed: the old standard, MBR, and the modern standard, GPT.

4 The Partitioning Standards: MBR vs. GPT

When you initialize a new disk in Windows, you choose between MBR and GPT. This choice determines the capabilities and compatibility of the drive.

4.1 MBR (Master Boot Record)

MBR is the legacy standard, introduced in the early 1980s. The “Master Boot Record” itself is a tiny section at the very beginning of the disk that contains two things:

  1. The Partition Table: Describes where the partitions are located.
  2. The Boot Code: A small program that the computer’s BIOS loads to start the operating system boot process.

MBR has significant limitations: - 2TB Size Limit: MBR cannot address storage space beyond 2 Terabytes. If you initialize a 4TB drive as MBR, you will only be able to use the first 2TB. - 4 Partition Limit: An MBR disk can have a maximum of four primary partitions.

4.2 GPT (GUID Partition Table)

GPT is the modern standard that replaces MBR and is part of the UEFI specification (the replacement for the traditional BIOS).

GPT offers major advantages: - Massive Size Limit: GPT supports drives of enormous size (in theory, up to 9.4 Zettabytes, which is millions of Terabytes). The 2TB limit is gone. - More Partitions: GPT allows for up to 128 primary partitions in Windows, eliminating the need for the complex scheme used by MBR. - More Robust: GPT stores a backup copy of the partition table at the end of the disk, providing redundancy in case the primary table is corrupted.

Rule of Thumb: Always use GPT for any new drive unless you have a specific, legacy reason to use MBR (such as booting on a very old, non-UEFI computer).

5 MBR Partition Types: A Legacy Structure

To work around the 4-partition limit of MBR, a special system was created:

  • Primary Partition: A standard bootable partition. You can have up to four of these.
  • Extended Partition: To get more than four partitions, you can designate one of your four slots as an “extended” partition. This special partition acts as a container.
  • Logical Partition: Inside the extended partition, you can create numerous logical partitions.

So, a common MBR setup for a multi-partition drive was 3 primary partitions and 1 extended partition holding several logical drives. This system is complex and is completely unnecessary with GPT.

6 How Partitions Enable the Boot Process

The partition style is directly tied to how the computer firmware (BIOS or UEFI) finds and starts the operating system.

Legacy Booting with MBR

  1. The computer’s BIOS powers on and performs a hardware check (POST).
  2. The BIOS looks at the first sector of the boot drive, loads the Master Boot Record (MBR) into memory, and executes its boot code.
  3. The MBR’s code scans the partition table to find the one marked as “active.”
  4. It then loads the boot code from that active partition’s own boot sector, which continues the process of loading the operating system.

Modern Booting with UEFI and GPT

  1. The computer’s UEFI firmware powers on and performs a hardware check.
  2. Instead of running code from the MBR, the UEFI firmware scans the GPT for a special, dedicated partition called the EFI System Partition (ESP).
  3. The ESP is formatted with a simple FAT32 filesystem and contains bootloader files (with a .efi extension) for all installed operating systems.
  4. The UEFI firmware loads the appropriate .efi bootloader file directly from the ESP to start the operating system.

This method is more reliable, faster, and more flexible than the MBR boot process.

7 Reflect and Review

ImportantReflection: 3-2-1

Now that you have reviewed this document, take a moment to reflect on your learning in your Microsoft Teams Student Notebook:

  • 3 advantages of GPT over MBR.
  • 2 components found in the Master Boot Record.
  • 1 question you still have about the EFI System Partition (ESP).
TipCheck on Learning

Answer these questions in your notebook to solidify your understanding:

  1. You are trying to format a new 6TB hard drive, but the operating system will only let you use 2TB of it. What is the most likely cause of this issue?
  2. To get around the 4-partition limit in MBR, you can use one ______ partition to hold multiple ______ partitions.
  3. What is the name of the special partition that a modern UEFI system uses to find and load the operating system’s bootloader files?
  4. True or False: A GPT disk is required to boot an operating system in modern UEFI mode.
Back to top