Leopard 10.5 MacOsX Snow Leopard

Symptoms

If your computer won't start up normally, or you have Kernel Panic (grey screen) you may need to use a disk repair utility to fix the issue. Mac OS X includes two utilities for this: Disk Utility and fsck. You can also use these even when your computer starts just fine but you want to check the disk for possible file system issues.

Note: If you're using Mac OS X 10.4 or later, you should use Disk Utility instead of fsck, whenever is possible.

If your computer is crashing frequently, displaying cryptic error messages, or exhibiting unusual behavior, you can use First Aid in Disk Utility as a first step to check for defects and, in many cases, repair them. If, however, your volume has suffered severe corruption, you may need to use other utility programs or repair methods.

Cure

Disk Utility is a program you can find in Applications/Utilities

Even if you are not getting errors, it is a good idea to run First Aid periodically. This preventive maintenance will help keep small problems from developing into bigger ones.

In some situations, file system errors may prevent your computer from starting up. This can occur after an improper shutdown, forced restart, or power interruption. If your computer shows any of the following symptoms on startup, use a disk repair utility:

Your computer partially starts but then displays a command line in a text-only environment. You may see the message, "file system dirty, run fsck." Below it, you'll see what's called a command-line prompt, indicated by a number sign (#), that allows you to type a command. If you see this, you'll need to run fsck from the command line (see "Use fsck," below).
Your computer starts but either it won't reach the login screen, or it may reach the login screen but not load the Desktop after you log in. However, you can start up in single-user mode.
If your computer exhibits either of the above issues, here are some things to try to get your computer back to starting up properly again. If you can't find the system discs that came with your computer, see "Use fsck," below.

fsck

Starting up in single-user mode or "fsck"

You should do this only if you are comfortable with UNIX and are confident that you know what you are doing.

Choose Apple menu > Restart or press the power button if the computer is turned off.
Press theHold down the Command (i.e. cloverleaf or Apple) and "s" keys as the system begins to boot / starts up.
This will drop you into a command line interface before the system has fully started up, giving you the chance to check/repair things before much of the normal OS X environment starts up.

Check the boot volume's file system, and repair if necessary (the "-y" means "Yes, go ahead and fix any problems you find"). Always do this first. Note that this may not be able to fix all problems in a single pass, so if it finds and fixes anything (it'll print "***** FILE SYSTEM WAS MODIFIED *****"), run it again, and keep running it until it comes back with something like "The volume Macintosh HD appears to be ok."

fsck -y

It is very important to write it EXACTLY as it appears here above,
with one space between the k and the -y

Note: In single-user mode, not all functions of Mac OS X are available. To use the Mac OS X interface you need to start up in the standard mode.

To switch back to the Mac OS X interface, type

reboot

and press the Return key. The computer restarts and you'll see the login dialog or the Mac desktop, depending on your login preferences.