Remove any external devices

If your Mac won’t start properly, try removing any external devices, except for your display, keyboard and mouse. If an external device isn’t working properly it can cause your Mac to have problems starting properly. If you find that the cause of problems is an external device, try attaching it to another Mac to see if you have the same problem.

Remove 3rd party internal hardware

Defective memory, or PCI card in your Mac can cause it not to boot properly. If you’ve removed any external devices from your Mac and still cannot boot, try removing any 3rd party memory or PCI cards that you have added to your Mac. If removing these components solves the problem, reinstall them one component at a time to find exactly which is the cause of the problem.

Boot from a CD

If your Mac doesn’t boot properly and you suspect a problem with Mac OS X, try starting your Mac from your Installation CD. From the Installation CD you can run diagnostics to check your hard disk for errors, and repair most of them. Also, if you can start your Mac from a CD, you have isolated the problem to either your hard disk, or some data on your hard disk.

Start Mac OS X in Safe Mode

Safe Booting forces a directory check of the boot volume and loads only required kernel extensions and Apple-installed startup items. To Safe Boot your Mac into Safe Mode, start your Mac and as soon as you hear the startup tone, press-and-hold the Shift key.

Start your Mac in FireWire Target Disk Mode

Starting your Mac in FireWire Target Disk mode will allow you to connect it to another Mac to run diagnostic utilities on your hard disk, or copy important files from one Mac to another. To start your Mac in FireWire hold the T key when starting your Mac. Your screen will turn blue with a yellow FireWire logo, when you see this you can connect your Mac to another Mac with a FireWire cable and your Mac will show up as a FireWire disk drive.

Reinstall Mac OS X

If you are unable to resolve a problem with Mac OS X, and you know that the problem isn’t hardware related, you might try reinstalling Mac OS X as a last resort. The best option when reinstalling is to erase your hard drive and install Mac OS X. Before doing this, make sure that you’ve backed up all of your important data to an external hard disk, CD or DVD, or another computer.

Using the Archive and Install option when reinstalling

If you are unable to back up your important information before reinstalling Mac OS X, use the Archive and Install option to install Mac OS X. This will back up your current installation and install a new operating system, while preserving your user preferences and most applications. After performing an Archive and Install you will need to reinstall some of your applications for them to work properly.