Mac Startup Disk Full? Try These Tips to Clear Space

You probably weren’t expecting to see the message, Your startup disk is almost full. The rest of the message isn’t that helpful, merely advising you to delete some files to fix the issue. 

What files should you delete, and why should you be worried about the startup disk almost being full anyway? Oh, and one other question: what's a startup disk?

Let's start with the easier of those questions.

What's a Startup Disk?

When you turn your Mac on, it will use one of its internal or external storage devices to start up from. The startup disk will contain the MacOS system software, which will be used to run your Mac while you’re using it. The Mac can have multiple disks that it can use to start up from, but for most users, the internal drive that came with their Mac is the default startup drive. If you ever see a flashing question mark when you start up your Mac, it may indicate that your Mac is having problems finding the startup drive.

How Full Is Too Full?

Having a startup drive that is almost full not only presents problems simply because you’re running out of storage space, it also impacts the performance of your Mac. Insufficient storage space quickly degrades how fast your Mac can start up; it also impacts the speed of launching and using apps. It won’t be long before you’re wondering why your Mac is so slow, and if you need to buy a new one to get some decent performance.

The good news is you don’t need a new Mac; instead, you just need to keep more free space on your Mac, at least 15 percent free for most users so it can continue to perform at its best.

With the easy questions out of the way, it's time to move on to the more difficult ones. 

Back-Up Before You Proceed

Before we get too far along, it's a good idea to point out that there are some files you should never remove. Most of these relate to the operating system, files needed by the OS to run. But there are others as well; files needed by your favorite apps, documents you need, media files that are important to you. There are few things worse than discovering you just deleted your only copy of last year's vacation pictures.

For that reason, before you begin searching for and removing additional files, perform a current backup of all your files before you proceed.

Which Files to Remove?

If you haven't been able to free up the needed space on the startup drive using common cleanup strategies, it’s time to dig a little deeper to find files to remove.

The Mac operating system can present an overview of which file types are taking up the most space on your Mac. There are two methods of accessing the storage graph that displays this information, depending on the version of the Mac operating system you’re using.

OS X Mavericks and earlier

1. From the Apple menu, select About This Mac.
2. In the About This Mac window that opens, click the More Info button.
3. The About This Mac window will expand and display a number of tabs in its toolbar. Click the item labeled Storage.

OS X Yosemite through current versions of the macOS

1. Use the Apple menu to select About This Mac.
2. In the About This Mac window that appears, select the Storage item from the windows toolbar.

Storage Graph

The storage graph displays all currently attached storage devices with Mac's startup disk listed first. The graph for each storage device displays the amount of space each file type is taking up on the disk. In looking at the graph, you may notice that you have a large collection of apps on your disk. Sorting through your apps and removing the ones you don’t use may help increase free space.

Another possibility is that the bulk of the space is being taken up by photos, movies, or other file types that you don’t wish to delete. If that’s the case, you should consider adding additional storage space using an external drive and moving some of the files to the new drive.

Managed Storage

Using just the storage graph can point you in the right direction, but it actually doesn’t make deciding which files to remove that much easier. Starting with macOS Sierra, Apple added Managed Storage, a collection of services and tools that make managing your storage needs a bit easier.

Managed Storage currently only works with files on the startup drive. To access the available storage utilities, bring up the storage graph as outlined above. Then:

1. Click the Manage button next to the startup drive’s graph.
2. The Managed Storage window will open, displaying a sidebar that shows how disk space is used by file categories.
3. Select the Documents category in the sidebar and the central section of the window will display all the document files you have stored on the Mac.
4. Across the top of the center section are three tabs: Large Files, Downloads, and File Browser. Select Large Files to display the documents sorted by size, largest first.
5. Scan the list and when you find a file you wish to remove, move the cursor over the file name and two small icons will appear. The first is an X that, when clicked, will delete the selected file. The other is a magnifying glass, which, when clicked, will display the file in the Finder for additional examination.
6. To delete an item, click the X (delete icon) next to the file name.
7. A sheet will drop down, asking if you really wish to delete the selected item. The sheet will also let you know how much space removing the item will free up, and that the removal of the item can't be undone. Click the Remove button to proceed.
8. The Downloads folder is often full of items that can be removed. Click the Downloads tab to view the content of the Download folder organized by age, with the oldest files displayed first. You can then use the same method as above and click the X to remove a selected file.

Once you’ve finished removing files, you should have been able to free up at least 15 percent of the space on the startup drive. If not, you may want to consider installing a larger drive or adding an external drive and moving some content to the new external to free up drive space.