Your Macintosh computer has a finite capacity for file storage. A full hard drive isn't merely an inconvenience; it can crash and potentially lose all your personal data. Getting the "Startup Disk is Almost Full" message means you must remove some files from your hard drive right away. Properly offloading data averts far larger problems down the road.
How to Determine Disk Usage
When your hard drive or solid state drive approaches capacity, a warning dialog box appears on the screen. Overall sluggishness is another sign of a full drive on your Mac. Highlight the drive on the desktop, which is named "Macintosh HD" by default, and click the "Command" and "I" keys to display a window that indicates how much space is used and how much is available.
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First Thing to Do
Empty the trash by clicking on the "Trash" icon in the dock and selecting "Empty Trash." This action could be all that is needed. Check how much free space you have when the Trash is empty.
Look for Problem Areas
In most cases, the "big three" file types that take up space on a Mac's hard drive are movies, music and photos. However, there are also other space hogs, including unused user accounts, downloaded files and high-resolution graphics files.
Archiving on an External Drive or DVD-R
Attaching an external USB drive allows you to archive seldom-used files. Drag the files from your Mac onto the external drive, wait for them to copy and then trash them on the Mac and empty the trash. You can also use DVD-R media to archive data, but that requires a DVD-R drive, something not all Macs possess.
Decide What to Archive or Trash
You can archive or delete anything inside your Downloads folder, anything inside your Documents folder except for the Microsoft User Data folder, and completed iMovie projects. Also, delete any unwanted music and movies in your iTunes library by highlighting the song or movie and clicking the "Delete" key. Inside the Applications folder, remove any outdated or unused applications by dragging them into the trash.
Moving the iPhoto Library
Your iPhoto library can be stored on an external drive. Drag your iPhoto library, usually called "iPhoto Library" and found inside your Pictures folder, onto the external drive. Double-click it when copying has finished. All subsequent launches of iPhoto open the library from the external drive. Delete the original library from your Mac's hard drive afterward.
Macs with Multiple User Accounts
If you have multiple user accounts on the Mac, repeat these processes for each user account. In addition, delete any user account that is no longer needed by opening "System Preferences" in the Applications folder, selecting "Users & Groups," highlighting the account to be deleted and clicking the minus sign.
Before deleting any items, perform a current backup of all data. Incorporate your external hard drive into your backup process to prevent the loss of archived files should that drive becomes corrupt.