Are your boot disk permissions set to allow access to no one? This will help

Occasionally users can accidentally change the permissions settings on their boot disk to deny access to everyone. This can cause your Mac to not boot properly. If this happens, follow these simple steps to restore normal permissions.

Apple Support Discussions user "macfredi" writes:

"I accidentally set de hard disk permission to no access to everyone. Yes, you guessed I cant start the computer! What can I do? "

Possible solutions
Try some of these solutions provided by Apple Support Discussions user "Niel":

A1. If the disk in question is your Mac OS X startup disk and your computer stops starting up at the blue screen, restart with the Command and S keys held down, and enter the following commands:

mount -uw /
chown root /
chmod 1775 /
exit

A2. If the disk in question has Mac OS X installed but isn't the current startup disk, open the Terminal in the /Applications/Utilities/ folder and enter the following:

sudo chown root "/Volumes/volumename/"
sudo chmod 1775 "/Volumes/volumename/"
sudo -k

You will be asked for your password when executing the first command; it will not appear in the Terminal window. Replace volumename with the disk's actual name. If you don't know what the disk's name is, choose Go to Folder from the Finder's Go menu, and enter /Volumes/ as the folder's path. Locate the item corresponding to that disk and drag it into the Terminal window; in this case, omit the quote marks and all text between them in the existing commands. Log out and back in.

A3. If the disk in question does not have Mac OS X installed, open the Terminal in the /Applications/Utilities/ folder and enter the following:

chmod 775 "/Volumes/volumename/"

Replace volumename with the disk's actual name. If you don't know what the disk's name is, choose Go to Folder from the Finder's Go menu, and enter /Volumes/ as the folder's path. Locate the item corresponding to that disk and drag it into the Terminal window; in this case, omit the quote marks and all text between them in the existing commands. If you get a message that you don't have permission to do this, proceed as indicated in situation 2 above, but use 775 as the number. Log out and back in.

Resources
Read this Apple Support Discussions thread about changing permissions on your startup disk.
Read the full post from ASD user "Niel".
Read this supplemental support thread if suggestions from the above post do not work.

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Resources

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