In modern versions of macOS Apple includes a bar chart inside the “About this Mac” menu which shows you how much space is used on your Mac’s drive. It even shows which categories of data are using the space, such as Apps, Music, Documents… And “System”. Unfortunately the System category is very broad and can include many different types of data, leading many Mac users to become perplexed as to why 100GB, 150GB or even 200GB of space on their computer is apparently being used by the “System”.
The built-in storage chart in macOS is similar to that of an iPhone, showing how much data is taken by apps (from the App Store), music (.mp3), movies (.mov) and documents (.pages). It isn’t very useful when it comes to more advanced types of files, such as those used for caches, application support, backups etc. This makes sense in a way as many Mac users wouldn’t understand what these files are for, and it wouldn’t be a good idea for them to start deleting data they don’t know the purpose of. Presumably this is why all these kinds of advanced files are bunched into the “System” category.
The DaisyDisk application (available via the Mac App Store or from the developer’s website) will allow you to drill down and find out exactly which files are using up how much space on your Mac. You may find that a lot of the data macOS is allocating to the “System” category is in-fact data generated by the user, or applications that the user installed, not the system. These include emails in your Outlook profile, Google Chrome browsing caches and Time Machine local backups. Removing these will not harm your Mac’s operating system and can be safely deleted, assuming you don’t need the data of course.
However what if DaisyDisk also shows a large amount of disk space taken by “Hidden Space”. You won’t be able to see exactly which files these are, as macOS will not allow DaisyDisk access to those files. If you find yourself in this situation then the files taking up the space may in-fact be system “cache” files managed by macOS. The macOS operating system is generally quite good at cleaning up after itself, so it’s likely that you won’t actually have to do anything to “fix” the issue. It’s common for a Mac to have 100-200GB of space taken up by the system and 20-50GB free, then when the user adds more files and space gets really low macOS will purge unneeded system files automatically to make room for more user files. This is much different to Windows which can just keep holding onto old system log files until the computer completely runs out of space and stops working properly. So as long as your Mac has at least a few GB of space free it may be best to just ignore the 100-200GB of space taken by the system and let macOS sort it out when needed.
If you ever do run out of space completely and need to squeeze a few more GB out of your Mac, check out our article How to clear space on your Mac for some more generic methods to get some more space back.