Computers rely on partitioned hard drives to store, retrieve, and process data. In modern computing, hard drives require both a boot partition and a system partition for proper functionality. Additional partitions can be created, and they're often used for storing and organizing personal files, but the boot and system partitions are found in nearly all modern systems today.

In some Linux installations, the boot and system partitions are one in the same. Conversely, most Windows installations use a hidden system partition in tandem with a visible boot partition. Although the boot partition does contain some important files, Windows also uses it for general storage. It's the boot partition that is usually assigned the "C:" label.

But what exactly are these partitions for? How are they used in various operating systems (OS) and, perhaps even more importantly, how can they be used in data recovery?

The Common Definition
Most operating systems, including Linux and Mac computers, use the common definition to separate the boot partition from the system partition. Under this definition, the boot partition is responsible for storing the boot loader. In a Linux installation, boot files are usually found at the "/boot/" location.

In this scenario, the system partition is responsible for storing the operating system folder and any supporting files or directories. Within most Linux distros, the OS folder is known as the "system root."

As mentioned, some Linux installations put boot loader files and system files on the same partition. This occurs when both the "/boot/" directory and the system root directory are both located on the same partition.

The Microsoft Definition
While the common definition of a system disk works great for Linux distros and Mac installations, the development team with Microsoft Windows actually maintains a very different definition.

According to Microsoft, the system partition, also known as a system disk or system volume, actually contains the boot loader. This is also where you'll find the system's boot sector. Conversely, the boot partition, also known as a boot volume, houses the operating system folder and the critical system files stored within.

In pre-Windows 7 installations, the boot and system partitions were one in the same. They were configured this way by default and given the "C:" drive, which is generally used to store personal files, too.

The release of Windows 7, however, featured a small updated to the boot and system partitions. Instead of including them within the same partition, Windows Setup now creates a separate system partition that is hidden by default. Now, the boot partition is designated as the "C:" drive. With this configuration, the latest versions of Microsoft Windows are able to support BitLocker for data protection.

Making Sense of the System Disk
As you can see, Microsoft's definition of boot and system partitions is the opposite of the common definition. Nonetheless, both still contain the necessary files and directories for booting up the machine and maintaining the OS – they're just handled a little bit differently. Although this can be confusing in some scenarios, the average user will hardly even notice a difference between the two.

You may read more about system disk in Wikipedia: Boot disk.