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The HDD (Hard Disk Drive) and the SSD (Solid State Drive) are two of the most common options today for storing and retrieving data. These are your "disks".

Hard Disk Drives are what a lot of people may have heard others call the "hard drive". This phrase is accurate for HDDs and SSDs - they're both "hard" (physical) drives inside of and connected to the system, and they're used for storing a lot of information.

Here's what an HDD looks like:

Older HDD

An older Hard Disk Drive

An SSD is more compact in its design and a lot "cleaner" looking:

Solid State Drive

Solid State Drive

They're much smaller as you can see below where I've included my car key fob and HDD for scale:

Solid State Drive scale

Solid State Drive, HDD and keyfob for scale

And there is a second form factor that an SSD comes in, called an M.2 based on a technology called NVMe:

Solid State Drive

M.2 NVMe Solid State Drive
(Dmitry Nosachev, CC BY-SA 4.0, via Wikimedia Commons)

Both the HDD and the SSD are often referred to as "drives" or "block devices". The term "drive" (probably) comes from the fact we call them drives in Windows, like "C: drive" or "Z: drive".

The term "block device" is more complicated, but in short it means a device on which a file system stores "blocks" of information. This is a complex topic and one we'll cover in a bit more detail when we look at file systems. For now, just keep the terms "drive" and "block device" in mind.