When using your Mac, you may regularly download, install, and create content, but then run into a warning that you need more free space on your hard drive. If this happens, you might also sense the system running a little slower in general, which can be frustrating to deal with.
If you encounter such a warning, then first check how much space on your hard drive is free. To do this, open a new Finder window and choose your hard drive on the Finder sidebar, or optionally in any Finder window press the Command key and then tap the up arrow until the displayed contents stops changing, and you will be left with a window showing your drives, optical discs, and the Network icon. You can then click your boot drive, which is by default named Macintosh HD.
With your boot drive selected, press Command-i to get information on it, and then expand the "General" section in the information window. In here, check the "Capacity," "Available," and "Used" statistics, to see how much space is used.
Note that these statistics are raw readings of your drive's usage, and should be accurate.
Next, go to the Apple menu, choose About This Mac, and then click "More Info." In the window that appears, click the "Storage" tab in the toolbar, and you should see a graphical representation of the space used on your drive. Compare the usage reported here to that in the information window you opened earlier.
Note that this graphical representation is derived from a Spotlight index of your drive, so if the total space used is significantly different than that reported in the information window, it suggests your Spotlight index may need to be rebuilt. To do this, add your hard drive to the Spotlight Privacy list in the Spotlight system preferences, and then remove it.
If the Information window shows your drive only has a few gigabytes of space left, then your drive is nearly full. However, first be sure the drive's formatting is not showing any errors. The space used on a hard drive comes from the format's database files, and if these are damaged then the free space on the drive can be incorrectly reported. Therefore, first open Disk Utility and check your hard drive for errors.
If there are errors in the formatting, then you can try fixing the boot drive by booting into recovery mode and then running Disk Utility to repair your boot drive; however, be sure to fully back up your system beforehand using Time Machine or similar full-system backup routine. If you cannot fix the drive with Disk Utility, then it may be best to format it and restore your entire system from your backup.
Finally, if you have recently migrated data from another drive, between application libraries (such as iTunes or iPhoto), or restored from a backup recently, or even under standard uses then you might have inadvertently created duplicates of many files on your system. Unfortunately, if this happens in program content libraries (again, iTunes and iPhoto), then sometimes the programs that handle them will not report the files in your library. Therefore, checking for duplicates in these libraries can be a bit daunting; however, it can be done by selecting items in the library and choosing the option to reveal them in the Finder. In this view, you can peruse them to see if a duplicate exists.
For example, in my iTunes library I have a number of songs that show in iTunes as only being one copy; however, if I go to the iTunes folder for many of them, there is clearly a second version of the song. While this may not appear to be much space used for a single song, it represents a large number of gigabytes collectively for all my iTunes songs.
From here, you can manage any duplicate files you have created, by deleting the ones not linked to your library.
Beyond application library contents, you can delve further into what might be using up your hard drive space by using a program like GrandPerspective or DaisyDisk to analyze your disk usage and give you a visual aid to what files and folders are the largest on your system.
Using programs like these, you might find an unused collection of movies that you might have inadvertently duplicated, or you might find a massive system log file (which sometimes can become gigabytes in size), or other collection of files that appears to be massive, and remove them.
These visual approaches for managing your hard drive's contents can also be beneficial for seeing what regular actions may be resulting in massive hard drive use. For instance, if you regularly take videos with your phone and synchronize them to your computer, then you might not realize the videos can take up many gigabytes, expanding your iTunes library to use a massive amount of your disk space.
At this point, it is up to you how to proceed. For large libraries you can move them to an external hard drive, or downsize them by deleting unwanted content, and for files scattered on your hard drive, you can back them up to an external drive and then delete them. However, regardless of what you do, you should clear enough space to have about 10 percent of your hard drive's capacity free, since free space on the drive is required to allow the system to run smoothly.
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