Repair Permissions — if your Mac slows down and things keep going wrong: Apple placed a utility on your Mac which fixes many problems. Your Mac uses a Unix file structure, and this gets corrupted. For example, you have file X in your documents' folder. Unix sees the direction to find this file as /(your user name)/documents/X.
Application unresponsive? Hold down the Command and Option keys together and press the Escape (esc) key – it's normally at top left of your keyboard. You'll get the Force Quit dialogue. Click on the troublesome app listed (it will be clear which one it is, it's marked 'no responding') and click the Force Quit button.
Unsaved changes will be lost but you should be able to reboot the app and keep working.
Application frozen and can't use mouse/trackpad/keyboard? Holding in the Mac's power button for six seconds or longer forces it to restart. This is a bit radical, but effective. Restarting flushes the RAM and a plain restart can solve many problems in itself.
CD, DVD stuck in the drive? Don't worry – restart, hold down the mouse button or trackpad-click. Keep it held down, and the disc should eject. Then you can let go.
You move it into a different folder. Unix lists the new path as /(your username) /documents/(different folder)/X. The forward-slash represents a folder, or (in Unix-speak) a Directory. But the previous unused path is remembered. Move the file a few more times, rename it, add to it – Mac OS has to sort through all these redundant paths to find the right one. This adds up to a slowdown, and multiply this out a few times and all those milliseconds add up to actual seconds and spinning beach balls.
Also, connections that Finder needs to find and link files can get corrupted or confused.
Solution: Open your *Utilities’ folder in your Applications’ folder, open Disk Utility, make sure the First Aid tab is selected (and definitely not Erase!), click once on the icon for your HD over on the left and click Repair Permissions, which finds all the places where all the files are, deletes all the incorrect paths and writes fresh new paths. Result – a faster, happier Mac.
*In Unix speak, your path is ‘Macintosh HD/Applications/Utilities/Disk Utility’.
OS X's Safe Mode Boot — From Lion on (so also Mountain Lion) users can use an interim step that verifies the directory structure of your hard drive at startup.
To start up in Safe Mode, shut your Mac down (choose Shut Down from the Apple Menu). When it's off, your Mac and immediately hold the Shift key on your keyboard down until you see a grey screen with an Apple icon and a progress bar beneath it. Safe Mode takes a while, so don’t be alarmed that you don’t see the desktop right away.
The progress bar indicates the operating system is verifying the directory structure of your startup volume. It will repair it, if necessary. It will also delete some of the startup caches that may also be preventing your Mac from starting successfully (these will be recreated fresh).
Once the desktop appears, you can access and run Disk Utility’s First Aid tool just as you normally would, as above. But when First Aid is finished, restart your Mac normally.
Not all applications and OS X features will work when you are booted into Safe Mode. Just restart afterwards for normal operation – you use this startup mode only for troubleshooting and not for running day-to-day applications.
This is Lion's answer to not supplying a startup disc, since Lion was a digital download.
Repair Disk: You may notice, in Applications>Disk Utility>First Aid, there’s a Repair Disk option. It’s greyed out and unusable on the hard drive your system is installed on. As an analogy, if you’re a heart surgeon, you can't operate on your own heart.
But If you have an external hard drive plugged in, you can select that on the left and it’s active. Use it to repair any external drives, for example Time Machine backup drives, extra hard drives and even thumb/USB drives, every now and again to make sure all’s good.
(I don’t bother with Verify. It takes as long as Repair, except all it does is tell you yes, you have problems – or not – without fixing them.)
To repair your System Disk: You have two options if Repair Permissions doesn't work and/or your internal hard drive is still causing problems:
For OS 10.6x (Snow Leopard) and versions of OS X before that, put in your original Mac OS install DVD (Lion doesn't come on an install DVD – see below), restart and immediately down the C key on your keyboard. Keep the key down until you can hear your Mac laboriously booting from the DVD. The system is booting from the OS on the DVD instead of your Mac (so it's slow).
In the installer that eventually appears, select ‘Use English for the main language.’ When this option appears, click the arrow button.
Now select ‘Disk Utility’ from the Utilities menu – in the First Aid tab, select your Macintosh HD on the left and you’ll see Repair Disk is an option using this method. Press Repair Disk, hope and be patient.
Now, if this tells you it can’t fix it, you may well do need to take your Mac to a tech, sorry. So I hope you have a backup...
For OS 10.7x (Lion and Mountain Lion) — Lion Recovery:
OS X Lion includes a built in set of utilities in a Recovery HD partition. To access these, restart your Mac and hold down the Command key and the R key (Command-R), and keep holding them until the Apple icon appears, indicating that your Mac is starting up. After the Recovery HD is finished starting up, you should see a desktop with a Mac OS X menu bar and a "Mac OS X Utilities" application window. Note: If you see a login window or your own desktop and icons, it is possible that you didn't hold Command-R early enough. Restart and try again. Apple has more info here
. Unix Fix Disk — This can be a life saver on all versions of OS X. It is brilliant if you can't find your original OS DVD, or you have Lion installed (which doesn't come with a System Disk), or you are on a plane or something.
Restart, and hold down the Command and S keys together. Keep them held down until the Mac starts up with a black screen with white text appearing.
You are now running your Mac as a Unix box.
When no more text appears, type 'fsck -fy' without the quotes. That's Eff Ess Cee Kay space hyphen Eff Wy. It may look rude, but it's the Unix Fix Disc command. Press Enter, and be patient as Unix checks and reorders your file directories and finds disk problems. It also, 99% of the time, fixes everything.
If all appears OK, after word 'root' type 'reboot' and press the Enter key on your keyboard. Your Mac will happily restart in OS X, everything fixed and working faster.
But if, when it's finished, if problems are listed, run fsck -fy again as sometimes this works.
But it lists a problem the second time, it probably can't fix it, so it's time to go and see a technician.
Apple Hardware Test — As Apple's support site says,
you can also use the Apple Hardware Test to see if anything physical is wrong with your hardware.
Disconnect all external devices except the keyboard, mouse, display, and speakers. If you have an Ethernet cable or external DVD drive, disconnect them.
Restart your computer, holding down the D key while the computer restarts.
After your computer restarts, you should see the Apple Hardware Test chooser screen. If you don’t, Apple Hardware Test may not be available on your computer. You may be able to start Apple Hardware Test from the Internet. Reconnect your computer to the network, and then restart your computer while holding down both the Option and D keys.
When the Apple Hardware Test chooser screen appears, select the language you want to use, and then press the Return key or click the right arrow button.
When the Apple Hardware Test main screen appears (after about 45 seconds), follow the onscreen instructions.
Apple Services Status Page —
If you are having trouble with an Apple online service (iCloud, Mail, iTunes Store, Mac App Store etc), before you panic, check Apple's Online Services Status Page
as sometimes the outage comes from there.