Bigger might be better, but you wouldn’t know it from looking at Mac hard drives. Thanks to expensive SSDs, the base storage for a new Mac is now only 128 GB, and expanded internal storage can be ludicrously expensive. To help you deal with this reality, macOS Sierra includes features to help save disk space. As part of this mission, macOS analyzes your files and dumps them into two buckets into two buckets: purgeable space and everything else.
What Does Purgeable Space Mean?
For macOS, “purgeable space” means “expendable files.” Specifically, it describes files that macOS has decided could be deleted, if necessary, to free up disk space. Purgeable data still exists as ones and zeros on your hard drive, but macOS has marked the files as available for deletion.
As of now, it’s not possible to see which files, exactly, the OS has marked as purgeable space. You also can’t clear the purgeable space yourself unless you resort to some Terminal hacking. But we do have a general idea of what can get marked as purgeable space, and we can make some tweaks with the storage optimization options.
What Sort of Files Does Sierra Consider Purgeable?
That sounds crazy—my computer will just delete some of my files? Well, it shouldn’t delete anything you want, and it definitely won’t delete the only copy of something.
Two main factors affect whether a file is purgeable: when the file was last accessed and if the file is backed up in iCloud. If the file can be downloaded from iTunes, Photos, or iCloud on demand, and it hasn’t been opened in a while, there’s a good chance it will get marked as purgeable. The idea is to keep redundant data from clogging up your hard drive when perfectly good copies are available in the cloud.
If you want to go full-bore into macOS-optimized storage, turning on iCloud Desktop and Documents vastly expands the potential pool of files that can be marked as purgeable. These folders probably contain most of your files, and once they’re backed up on iCloud, any of these files could be a candidate for purgeable space. User files that are not backed up to iCloud should never be marked as purgeable space.
Other candidate files include special cases like TV shows and movies in your iTunes library that you’ve already watched, foreign language dictionaries and infrequently used fonts.
Exploring Optimized Storage
It doesn’t end at purgeable space. Sierra also offers a few features to help you better manage your storage space.
By design, Sierra’s storage optimization operations are hidden from the user’s view. You shouldn’t need to take any direct action to benefit from the basics, but you can take a look under the hood to get some more control.
If you open up “About This Mac” under the Apple menu and click on the Storage tab, you’ll see a breakdown of your disk storage. Click the button labeled “Manage” to open up a System Information window.
There’s a couple things going on in the resulting window, which will allow you to tweak your storage settings.
On the left, you have a few categories of files, including Applications, Documents and Mail, along with the disk space each category takes up.
Clicking on the Documents tab, for example, will show you most of your files, sorted by size. Based on our tests, it seems like “Documents” includes the Documents, Downloads, and Desktop folders.
This is probably the most useful feature of the bunch. It helped me locate some long-lost, disk-hogging audio files, which was great. But not all of these menu options are as useful. Clicking on GarageBand, for example, tells you exactly nothing.
Optimized Storage Recommendations and Options
Under the “Recommendations” tab (which is the default view) you have a few other options.
Store in iCloud
The first is called “Store in iCloud.”
While the description reads “store all files in iCloud,” “all” is a bit of an overstatement. This switch enables iCloud Desktop and Documents. That backs up the content of your Desktop and Documents folders to iCloud. Once a file is safely stored iCloud, and it hasn’t been accessed in a while, it gets the purgeable space flag. This means it can be removed from your hard drive, but a copy will remain safe in iCloud, ready to be downloaded as soon as you need it.
The same goes for your Photos library. Any full-res photos that also exist in iCloud can be flagged as purgeable space as well, but optimized JPGs will stay on the system. This is almost identical to what happens when you click the “Optimize Photos” toggle on your iPhone.
The second is somewhat inaccurately labelled “Optimize Storage.” It removes any iTunes movies and TV shows that you’ve already watched.
These videos can be re-downloaded and are still marked as purchased, but the local version is removed from your hard drive.
You might be sensing a trend here: all the “optimize” options are about removing files from your local hard drive that exist somewhere else.
Empty Trash Automatically
The third, “Empty Trash Automatically,” will automatically delete files that have been in the Trash for more than thirty days.
Finally, clicking “Review Files” under “Reduce Clutter” opens up the Documents tab that we saw before.
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