A
There is not enough free space on the startup volume. It may be running out of space intermittently.
For information about the Other category in the Storage display, please see this support article. If the display seems to be inaccurate, try rebuilding the Spotlight index.
Empty the Trash if you haven't already done so. If you use iPhoto, empty its internal Trash first:
iPhoto ▹ Empty Trash
In Photos:
File ▹ Show Recently Deleted ▹ Delete All
Do the same in other applications, such as Aperture, that have an internal Trash feature. Then restart the computer. That will temporarily free up some space.
According to Apple documentation, you need at least 9 GB of available space on the startup volume (as shown in the Finder Info window) for normal operation—not the mythical 10%, 15%, or any other percentage. You also need enough space left over to allow for growth of the data. There is little or no performance advantage to having more available space than the minimum Apple recommends. Available storage space that you'll never use is wasted space.
When Time Machine backs up a portable Mac, some of the free space will be used to make local snapshots, which are backup copies of recently deleted files. The space occupied by local snapshots is reported as available by the Finder, and should be considered as such. In the Storage display of System Information, local snapshots are shown as Backups. The snapshots are automatically deleted when they expire or when free space falls below a certain level. You ordinarily don't need to, and should not, delete local snapshots yourself. If you followed bad advice to disable local snapshots by running a shell command, you may have ended up with a lot of data in the Other category. Ask for instructions in that case.
See this support article for some simple ways to free up storage space. A common waste of space is old iTunes backups of mobile devices. As illustrated in this support article, select the Devices tab in the iTunes preferences window, select the backups you want to delete, and click Delete Backups. Keep at least one backup of each device that you still use.
You can more effectively use a tool such as OmniDiskSweeper (ODS) to explore the volume and find out what's taking up the space. You can also delete files with it, but don't do that unless you're sure that you know what you're deleting and that all data is safely backed up. That means you have multiple backups, not just one. Note that ODS only works with OS X 10.8 or later.
Deleting files inside a photo or iTunes library will corrupt the library. Changes to such a library must be made from within the application that created it. The same goes for Mail files.
Proceed further only if the problem isn't solved by the above steps.
ODS can't see the whole filesystem when you run it just by double-clicking; it only sees files that you have permission to read. To see everything, you have to run it as root.
Back up all data now.
Install the app in the Applications folder as usual. Quit it if it's running.
Triple-click anywhere in the line of text below on this page to select it, then copy the selected text to the Clipboard by pressing the key combination command-C.
security execute-with-privileges /A*/OmniDiskSweeper.app/*/M*/* 2>&-
Launch the built-in Terminal application in any one of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad and start typing the name.
Paste into the Terminal window by pressing command-V. You'll be prompted for your login password.
The application window will open behind other open windows. When you scan a volume, the window will eventually show all files in all folders, sorted by size. It may take a few minutes for the app to finish scanning.
I don't recommend that you make a habit of doing this. Don't delete anything as root. If something needs to be deleted, make sure you know what it is and how it got there, and then delete it by other, safer, means. When in doubt, leave it alone or ask for guidance.
When you're done with the app, quit it and also quit Terminal.
B
The symbol cache is very large. It stores data used in logs and crash reports, and it may have grown because it's corrupt. The cache is created automatically, and if deleted, it should be recreated when you restart the computer. Initially, at least, it will be a lot smaller.
I've seen one unverified report that a system became unbootable when the cache was deleted. That would have been due to a corrupt installation of OS X, but you should be prepared to reinstall if necessary. There is no reason to panic as long as you have good backups.
Back up all data before proceeding.
Please triple-click anywhere in the line below on this page to select it:
/System/Library/Caches/com.apple.coresymbolicationd
Right-click or control-click the highlighted line and select
Services ▹ Reveal in Finder (or just Reveal)
from the contextual menu.* A folder should open with an item named "com.apple.coresymbolicationd" selected. Move the selected item to the Trash. You may be prompted for your administrator login password. Restart the computer and empty the Trash.
*If you don't see the contextual menu item, select the line below and copy it to the Clipboard by pressing the key combination command-C:
/System/Library/Caches
In the Finder, select
Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You may not see what you pasted because a line break is included. Press return.
A folder named "Caches" should open. Inside it is a subfolder named "com.apple.coresymbolicationd". Move that subfolder to the Trash, restart, and empty.
C
These system modifications may do something useful to you, but any or all could be contributing to the problem:
DisplayLink
Fitbit Connect
If you want to keep them, do a trial removal to find out whether they're at fault, and refer to the developers for support if they are. Some or all may need to be updated.
Any third-party software that doesn't install from the App Store or by drag-and-drop into the Applications folder, and uninstall by drag-and-drop to the Trash, is a system modification.
Whenever you remove system modifications, they must be removed completely, and the only way to do that is to use the uninstallation tool, if any, provided by the developers, or to follow their instructions. If the software has been incompletely removed, you may have to re-download or even reinstall it in order to finish the job.
I never install system modifications myself, and except as stated in this comment, I don't know how to uninstall them. You'll have to do your own research to find that information.
Here are some general guidelines to get you started. Suppose you want to remove something called “BrickMyMac” (a hypothetical example.) First, consult the product's Help menu, if there is one, for instructions. Finding none there, look on the developer's website, say www.brickmymac.com. (That may not be the actual name of the site; if necessary, search the Web for the product name.) If you don’t find anything on the website or in your search, contact the developer. While you're waiting for a response, download BrickMyMac.dmg and open it. There may be an application in there such as “Uninstall BrickMyMac.” If not, open “BrickMyMac.pkg” and look for an Uninstall button. The uninstaller might also be accessed by clicking the Customize button, if there is one.
Back up all data before making any changes.
You will generally have to restart the computer in order to complete an uninstallation. Until you do that, there may be no effect, or unpredictable effects.
If you can’t remove software in any other way, you’ll have to erase and install OS X. Never install any third-party software unless you're sure you know how to uninstall it; otherwise you may create problems that are very hard to solve.
Trying to remove complex system modifications by hunting for files by name often will not work and may make the problem worse. The same goes for "utilities" such as "AppCleaner" and the like that purport to remove software.
D
Please back up all data before proceeding.
Triple-click anywhere in the line below on this page to select it:
/Library/ScriptingAdditions/SIMBL.osax
Right-click or control-click the highlighted line and select
Services ▹ Reveal in Finder (or just Reveal)
from the contextual menu.* A folder should open with an item selected. Move the selected item to the Trash. You may be prompted for your administrator login password. Log out or restart the computer and empty the Trash.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select
Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You may not see what you pasted because a line break is included. Press return.
E
If there's no improvement after all the above steps, disconnect the external display and run without it for a while.
F
From the menu bar, please select
▹ System Preferences... ▹ Network
If the Network preference pane is locked, click the lock icon in the lower left corner and enter your password to unlock it. Then click
Advanced... ▹ TCP/IP ▹ Configure IPv6: Automatically
If that's already the setting, change it to something else, then change it back. Click OK and then Apply.