A traditional hard disk drive (HDD)

  • How does a hard drive work and what's the difference between a HDD and SSD
  • What is good about a HDD and what is good about an SSD
  • Installing and upgrading an SSD to your Mac

Picking a hard drive used to be fairly easy, you’d get the biggest one you could afford. And possibly pick from one of two speeds: 5,400 RPM and 7,200 RPM (although faster and slower speeds are available).

These days things are a lot more complex. New SSD (solid-state drives) sit alongside HDD drives and thanks to falling prices they’re no longer an expensive rarity for rich business execs and computer fanatics.

Tech companies have also figured out how to combine SSD technology with traditional HDD technology to create hybrid drives with the best of both worlds. Apple has one of these called the Fusion Drive.

See: Macworld Storage Reviews and Hard Drive reviews

On top of that hard drives are sold in a myriad of different sizes, ranging from 3.5in to 2.5in for laptops, and even 1.8in for smaller devices. On some Apple products the hard drive can be replaced, on others it’s welded onto the circuit board. Meanwhile external options are available and Apple’s new Thunderbolt technology is so fast that it can keep up with any of the latest, fastest hard drives.

When it comes to hard drives it’s a minefield out there. This feature will help you understand and choose the right drive.

Read next: The best SSD drives for Mac | SSD buying guide 

How does an HDD drive work?

A traditional hard drive works pretty much like a miniaturized record player. Imagine the record is coated in a metal that can be magnetized: that’s the disk called the ‘platter’ and it spins around. The magnetic material enables it to store data (0s and 1s) as magnetized ‘on’ or ‘off’ blocks. Above the platter is an arm called the ‘actuator’ that reads these magnetized blocks as binary code (0s and 1s) or writes to the disk by setting the magnetic blocks on or off.

The faster the platter spins the quicker the hard drive works, which is why it’s measured in RPM (revs per minute) like a car. But the actuator has to be able to keep up with the speed, which is why they’ve got better over time.

There’s two ways of making the hard drive bigger, to have a bigger platter or to create smaller blocks of data on the platter. Both have happened over time which is why there are different hard drive sizes and units have become smaller over time.

Also read: How to chose best storage: SSD or Hard Drive, types of storage compared

Anatomy of a Hard Disk Drive

Image: Wikipedia

What’s so good about HDD?

The great thing about traditional hard drive technology is that it’s cheap. You can pick up this 1TB 2.5in hard drive from Amazon for just £54. That’s just 5p per GB, a ridiculously cheap amount compared to SSD drives (or even hard drives from years gone by).

Compare this to the popular Crucial M4 SSD which costs £130 for 256GB.  That’s almost 50p per GB, or 10 times as much. Given that an SSD costs ten times as much as a HDD you’d expect it to deliver something pretty special, right? The good news is that it does.

How does an SSD work?

Even though they’re often referred to as disks, Solid State Drives (SSDs) have no spinning platter like a traditional hard drive. Instead they act more like the RAM inside your computer (except they retain the information when the power is off). This is called NAND Based Memory (NAND isn’t an acronym incidentally, but a term used in logic).

Data is written to electrical blocks that are set to on and off (for 0s and 1s). There’s no magnetic or electro mechanical part. Modern drives also integrate DRAM with the Flash memory to further improve performance, and to cache data in case of a power failure.

Solid State Disks (SSD) run much faster

What’s good about SSD?

Speed is the number one factor to consider when picking an SSD drive. In testing a typically hard drive will return somewhere between 50-100Mbps depending on the speed and quality of the drive. With an SSD you can expect that to rise to somewhere between 300MBps and 500Mbps. So it’s roughly going to be between 5x to 10x faster at reading data. This makes a huge difference when running your Mac: opening documents, starting programs and even booting up all happen much faster. You don’t have to watch icons bouncing in the dock, and you’ll see far less of the spinning rainbow ‘waiting’ icon. Going back to a regular hard drive after used an SSD for a while is extremely difficult.

Because SSD drives do not have a spinning platter they are also completely silent in operation. The only sound you’ll hear from your Mac is the sound of the fan in use, which is often also very quiet in most Macs. No more clicking and whirring.

Is a HDD more reliable than an SSD

SSD drives are assumed to be more reliable than HDD drives. Because they do not have a spinning platter, or motor, or moving actuator arm it is generally thought that there’s less moving parts that are likely to go wrong. Of course this is weighted against the fact that HDD development is many years in, and SSD technology is being rapidly improved. Many SSD drives have shown errors or problems, Crucial had trouble with memory leaks in its M4 model (although this was fixed with a Firmware update.) There’s little reliable information about long-term reliablility of SSDs, but the general assumption is that they’ll last longer.

What is a Fusion Drive?

A Fusion Drive is Apple’s branded hybrid drive that mixes the best of SSD with HDD technology. Inside the Fusion Drive is a SSD NAND memory storage block, and a HDD platter. Where it gets clever is the integration with the Mac OS X operating system. Mac OS X quickly figures out what files, applications and data blocks you use on a regular basis and moves these to the speedy SSD, meanwhile the big files that you might only access a few times (say a movie file) is moved to the HDD. So the Mac mostly runs as quickly as a pure SSD drive expect for when you need those big files.

See: It’ll give your Mac a boost but is the Fusion Drive a rip-off?

And it enables you to have a huge hard drive at a good(ish) price. A 27in iMac has a 1TB HDD for £1,499. A 1TB Fusion Drive costs an additional £200. You can’t even get a 1TB SSD drive, the closest you’ll get is a 768GB SSD drive that costs a whopping £720 more.

See: Crucial M500 offer close to 1TB of storage space

The shame with a Fusion Drive is that there’s no way to add one to an older Mac, you can only pick it up with the latest iMac and Mac mini. However we do consider it well worth the investment.

Installing an SSD into a Mac

Whether or not you can upgrade a HDD to an SSD in a laptop depends on the model you’ve purchased. Currently the MacBook Pro 15-inch, Mac mini and iMac 27-inch are the only models that you can swap out the hard drives on (these will be joined with the new Mac Pro later in the year). The MacBook Air and newer MacBooks have their memory fixed to the motherboard, and the smaller iMac’s hard drive is inaccessible.

See: New Mac Pro could offer up to 8GB SSD and New iMac hard drive upgrades ‘difficult’

If you have an older Mac you can check if the hard drive can be upgraded to an SSD. A good place to look on information regarding the upgrade, and how to about can be found on the iFixIt.com website.

If you can’t install an internal SSD drive then you could consider a new SSD Thunderbolt powered external drive, like this Buffalo MiniStation Thunderbolt. This enables you to expand your storage levels without sacrificing on speed.

If you're interested in adding an internal SSD drive to your Mac, then read this: Best SSD to upgrade Mac, internal SSD options  Also read: Best hard drive for Mac and Best SSD for Mac