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  1. Computer Skills
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How to Keep a Mac's Hard Drive Clean


Aside from a few iMac and Mac mini models, every new Mac comes with an SSD as standard. This is great if you love having a blazing fast computer, but not so good if you want to be able to store hundreds of movies on it without taking out a second mortgage.

For a few years, large HDDs were the standard so you just didn’t have to worry too much about space constraints. Now, you’re lucky to have more than 500 GB of usable space to play with. 

As well as drives getting smaller, files are getting bigger. Things like HD, or high definition, movies and photos from DSLRs take up massive amounts of storage. I'll show you how you can take control of a Mac’s hard drive space.

Back Up Everything

Before going all trigger happy with the delete key, it’s important to make sure you can’t accidentally erase anything important. Run the Mac through its usual backup routine.

If you don’t have a backup plan set up, you should check out and implement the tutorials below before continuing with this one.

Killing the Space Hogs

Most of the files on any Mac don’t actually take up that much hard drive space. You can delete all the Word documents you like but you’re never going to free up more than a few hundred megabytes. On the other hand, a single Photoshop file can take up five or six gigabytes. 

Before deleting anything you need to identify the space hogs that are worth removing.

Not all big files are easy to find with the file system. User caches, iOS updates, backups and temporary files can take up a huge amount of space. 

To make sure you find everything, you need to use a specialised app like Daisy Disk or Clean My Mac.

daisy disk
Daisy Disk shows what files and folders use the most space.

Daisy Disk, which costs $9.99, takes a hands off approach to managing your SSD. It uses an attractive graph to break down what files and folders are taking up the most space. You can then drag any you want to get rid of to a collection bucket and delete them.

Clean My Mac costs $39.95, a lot more than Daisy Disk, but it also does a lot more. It can scan your disk and identify various different forms of System Junk as well as big files. If you want an app that will find and delete all the caches for you, it’s the one to use.

Keep, Store, Delete

Once you’ve identified the worst space hogs, it’s time to deal with them. You have three choices with each one: keep it, store it or delete it.

For any file you want to have on the Mac, the only choice is to just keep it where it is. For example, even though my working Lightroom catalogue is huge, I keep it on my Mac because I need to have access to it.

Photos are by far the biggest space hog on my Mac.

For the files you want to keep around but don’t need to have immediate access to, you should set up a storage system. 

A Network Attached Storage (NAS) device, desktop computer or even just a few spare external hard drives will work. Move the files to their new location and then delete them from the Mac. Make sure to back up the storage locations after you move data to them. This is what I do with any older photos.

For the files you don’t need anymore and won’t need in the future, delete them. You can use Daisy Disk or Clean My Mac to do it for you, or just throw them in the Trash from Finder. I do this regularly with Photoshop caches which can grow to 30 or 40 GB if I’m working on some big projects.

Dealing With Duplicate Files

While Daisy Disk or Clean My Mac can deal with the most obvious space hogs, there’s one area where they fall down: finding duplicates.

At one point, I had three copies of the same 15 GB photo library sitting on my Mac. 10% of its storage space was taken up with just a few hundred images. 

Most people won’t have a duplicate problem quite as bad as this but, if you move files around and forget to delete the originals, it’s easy to waste a surprising amount of space with the same few files.

The free version of Gemini will work for most people.

Gemini, from the same developer as Clean My Mac, makes dealing with duplicates easy. It costs $19.95 but there is a free trial. The main difference is that the full version of Gemini will delete files for you while with the trial you’ll have to go in and do it yourself. If you’re only trying to get rid of a few files, the free version will work perfectly for you.

Securely Erasing Data From a SSD

While everything above will work fine if you’re just deleting files to create space, if you’re deleting files for security reasons things get a bit more awkward. It also depends on whether you’re trying to erase a whole drive or just a handful of sensitive files.

The easiest truly secure, 100% guaranteed way to wipe an entire hard drive uses a drill bit and a lot of centripetal force. Unfortunately, if the drive still works after you’re done, you’ve messed up. 

To keep it usable, you can overwrite the entire drive with random data twice. This should work as long as the drive controller doesn’t use a weird data allocation strategy. One drawback is that it can reduce the lifespan of the drive.

disk util
Disk Utility can't securely overwrite SSDs. You need to use the Terminal to do it.

To overwrite an SSD with random data, connect it to your Mac. If it’s another Mac’s primary drive, use Target Disk Mode. Open Terminal and run the command diskutil randomDisk 2 /dev/diskNumber.

Securely deleting a handful of sensitive files is next to impossible without overwriting the entire drive. SSDs don’t store data on a magnetic platter like HDDs do. The physical location of data on an SSD changes regularly. 

With a HDD, you can instruct the Mac to overwrite the area of the platter where the sensitive data is stored. With an SSD, doing this will flag the area where the data is stored as invalid, however, the new data will be written to a different area rather than over the top of the old data. 

Over time the SSD will cycle back and overwrite the old data but you have no control of the process. In the meantime, if someone gets access to the drive they can theoretically read any flagged data that hasn’t yet been overwritten.

To avoid this situation, the best solution for sensitive information is to keep it encrypted. That way you can delete it as normal and it will be impossible for someone else to recover it, unless they some how end up with the encryption keys.

Wrapping Up

SSDs are awesome but the lack of storage space is a major trade off. Fortunately, prices are falling and drives are getting bigger so in the next few years we’ll reach a point where there's no longer the need to worry about storage space. Until then though, you’ll have to occasionally free up space on a Mac.

In this tutorial I’ve looked at how to do just that. I’ve also considered some of the issues with securely erasing files from an SSD. If you’ve any questions or solutions to the issues I raised, please post them in the comments below.

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