Upgrading a Mac's hard drive is one of the most popular Mac DIY projects. The smart, savvy Mac buyer will usually purchase a Mac with the minimum hard drive configuration offered from Apple, and then add an external hard drive or replace the internal drive with a larger one when needed.
Of course, not all Macs have user-replaceable hard drives. But even closed Macs can have their drives replaced, by an authorized service provider, or by an intrepid DIYer, with readily available replacement guides that can be found here and elsewhere on the Internet.
When to Upgrade a Hard Drive
The answer to the question of when to upgrade may seem simple enough: when you run out of space.
But there are other reasons to upgrade a hard drive. To keep a drive from filling up, many individuals keep deleting less important or unneeded documents and applications. That's not a bad practice, but if you find your drive getting close to 90% full (10% or less free space), then it's definitely time to install a larger drive. Once you cross the magic 10% threshold, OS X is no longer able to optimize disk performance by automatically defragmenting files. This can lead to overall reduced performance from your Mac.
Other reasons to upgrade include to increase basic performance by installing a faster drive, and to reduce power consumption with newer, more energy-efficient drives. And of course if you're starting to have problems with a drive, you should replace it before you lose data.
Hard Drive Interface
Apple has been using SATA (Serial Advance Technology Attachment) as a hard drive interface since the PowerMac G5. As a result, just about all of the Macs currently in use have SATA I or SATA II hard drives. The difference between the two is the maximum throughput (speed) of the interface. Luckily, SATA II hard drives are backward compatible with the older SATA I interface, so you don't need to concern yourself about matching the interface and drive type.
Hard Drive Physical Size
Apple uses both 3.5-inch hard drives, mainly in its desktop offerings, and 2.5-inch hard drives, in its portable lineup and the Mac mini. You should stick with a drive that's the same physical size as the one you're replacing. It's possible to install a 2.5-inch form factor drive in place of a 3.5-inch drive, but it requires an adapter.
Types of Hard Drives
While there are many sub-categories for drives, the two prominent categories are platter-based and solid state. Platter-based drives are the ones we're most familiar with, because they've been used in computers for data storage for a very long time. Solid state drives, usually referred to as SSD, are relatively new. They're based on flash memory, akin to a USB flash drive or the memory card in a digital camera. SSDs are designed for higher performance, and have been mated to SATA interfaces, so they can work as drop-in replacements for existing hard drives.
SSDs have two chief advantages and two chief disadvantages over their platter-based cousins. First, they're fast. They can read and write data at very high speeds, faster than any currently available platter-based drive for the Mac. They also consume very little power, making them a great choice for notebooks or other devices that run on batteries. Their chief disadvantages are storage size and cost. They're fast, but they're not large. Most are in the sub-150 GB range, with 64 GB or less being the norm. If you want a 500 GB SSD, be prepared to spend over $1,500. The 64 GBs are no bargain, either, easily topping $200.
But if you crave speed (and budget isn't a deciding factor), SSDs are impressive. Most SSDs use the 2.5-inch form factor, making them plug-in replacements for the MacBook, MacBook Pro, MacBook Air, and Mac mini. Other Macs need an adapter for proper mounting.
Platter-based hard drives are available in a variety of sizes and rotational speeds. Faster rotation speeds provide faster access to data. In general, Apple uses 5400 RPM drives for its notebook and Mac mini lineup, and 7400 RPM drives for the iMac and Mac Pro. You can purchase notebook hard drives that spin at the faster 7400 RPM as well as 3.5-inch drives that spin at 10,000 RPM. These faster spinning drives use more power, and generally have smaller storage capacity, but they do provide a boost in overall performance.
Installing Hard Drives
Hard drive installation is usually pretty straightforward, though the exact procedure for accessing the hard drive itself is different for each Mac model. The method ranges from the Mac Pro, which has four drive bays that slide in and out, no tools required; to the iMac or Mac mini, which can require extensive disassembly just to get to where the hard drive is located.
Because all of the hard drives use the same SATA-based interface, the process for changing out a drive, once you gain access to it, is pretty much the same. The SATA interface uses two connectors, one for power and the other for data. The cables are small, and easily maneuvered into position to make connections. You can't make the wrong connection, since each connector is of a different size and won't accept anything but the proper cable. There are also no jumpers to configure on SATA-based hard drives. This makes changing out a SATA-based hard drive a simple process.
All Macs except the Mac Pro have temperature sensors attached to the hard drive. When you change out a drive, you need to reattach the temperature sensor to the new drive. The sensor is a tiny device attached to a separate cable. You can usually peel the sensor off the old drive, and just stick it back to the case of the new one. The exceptions are the late 2009 iMac and 2010 Mac mini, which use the hard drive's internal heat sensor. With these models, you need to replace the hard drive with one from the same manufacturer or purchase a new sensor cable to match the new drive.
Go Ahead, Upgrade
Having more storage space or a higher performing drive can make using your Mac much more fun, so grab a screwdriver and have at it.