1. A flashing ? disk or folder at startup that doesn't go away.
2. A monochromatic (single color) screen at startup.
A black screen should not be considered the same as a monochromatic screen as it may indicate a power or PRAM issue. When it is obviously not due to bad power cord, or improper plug being connected to a machine that is over four years old may indicate the PRAM battery/capacitor needs replacing.
Under four years old, if the machine won't startup, resetting the PRAM can help.
The PRAM can help a flashing ? disk problem as well. In a pinch, resetting the SMC can help without affecting the PRAM.
Note if you have a PowerPC Mac (pre-2006), resetting the SMC or PMU directions may vary by model (read each article and if you have trouble identifying your Mac, ask on this board first):
3. Slow behavior of the computer when no "maintenance" tools are installed, and the issue is not related to internet hardware misbehaving or outdated peripheral drivers, or an overflowing hard drive (a hard drive over 85% full may experience a slowdown). Maintenance tools which affect system cache should be removed first to eliminate them as a potential cause. Mackeeper is the worst, and needs its own removal directions.
If things appear to be stuck, you can try a command-option-escape key sequence to force quit an offending application. Command is often the Windows or Apple logo key, and Option is often the Alt key on other keyboards.
4. Kernel panics. Kernel panics may happen due to hardware or driver, or directory issues. If they don't go away after repairing the directory, you can isolate the hardware or driver issues next.
5. System software that is supposed to be compatible, unexpectedly quitting without any "maintenance" tools installed.
If your data isn't backed up, and you are experiencing any of the above symptoms that won't let you get to a point where you can backup, see my data recovery FAQ.
To repair the directory, if you are running Mac OS X 10.7 or later:
1. Boot with command-R (command also has the cloverleaf or Apple logo on the key, or the Windows logo key may sometimes work on a wired keyboard, though ideally you should have an Apple, MacAlly, or Logitech keyboard with the Apple logo on it), and select Disk Utility. Select First Aid, and hit repair disk.
If repairing disk doesn't work after repeating several times, or you get an error message, you'll need to purchase Alsoft Disk Warrior. If you can't get to Disk Utility, try the next suggestion for 10.6.8 and earlier without the original installer discs. Quit from Disk Utility when done. Do not proceed with any installation.
If you do not have the original installer discs, and are running Mac OS X 10.6.8 or earlier (Macs older than July 20, 2011 will have installer discs you should normally be able to get from AppleCare if you are missing them).
or the Windows logo key may sometimes work on a wired keyboard, though ideally you should have an Apple, MacAlly, or Logitech keyboard with the Apple logo on it).
At the prompt type:
Followed by return key. Repeat running /sbin/fsck -fy followed by return key until the system says it is repaired. If it tells you it can't repair it, then
get Alsoft Disk Warrior. If it can repair it, type:
Followed by return key. Then type
followed by return key. It should boot normally.
If you have the original installer discs, use the startup manager to boot them if they won't boot normally.
3. After getting to the installer screen, select the language, but do not proceed with installation. Look in the menus of the installer for Disk Utility.
Open Disk Utility and select First Aid. Then select the hard drive in question, and then select Repair Disk. Repeat until it says it has been repaired. If it can't repair, get Alsoft Disk Warrior. Quit from Disk Utility and restart the Mac.
When Disk Warrior can't repair the hard disk, chances are it probably needs replacing.
Note: the Repair Permissions option you find on Disk Utility has nothing to do with the directory. However it can help if you have the same operating system on your Mac as the disc that has that Disk Utility program. Some installers that use administrative passwords can corrupt permissions and affect booting.
You can use the startup manager mentioned earlier to boot those installer discs to repair permissions. But remember, no Mac can use the installer disc from another Mac of a different age, or model name. So be sure to properly identify your Mac before attempting to use an installer disc. For more on Repair Permissions, see Apple's article on it.