Q: intel imac wont start after changing permission

What you did you should never ever do, especially when you don't understand the consequences. You may be able to fix things by doing the following:

Boot into single-user mode and at the prompt enter these commands pressing RETURN after each:

mount -uw /

chown root:admin /

chmod 1775 /

reboot

After rebooting Set the following Directory permissions.  You can paste these lines or enter by hand, but be careful to enter them correctly.

sudo su

Enter your admin password when prompted.  It will not be echoed.

chown root:admin /Applications

chmod 0775 /Applications

chown root:admin /Library

chmod 1775 /Library

chown root:admin /System

chmod 0775 /System

chown root:admin /Users

chmod 0775 /Users

chmod -R -N /Applications

chown -R :admin /Applications/*

I suggest you first print these instructions out using a large mono-spaced font so you can read them correctly. There are spaces in the lines that may not be easily observed.

My apologies for the inept instructions.

Restart normally. Open Terminal in the Utilities folder. Paste the commands in the Terminal application to avoid typos:

sudo su

Press RETURN. Enter your admin password when prompted.  It will not be echoed to the screen. Press RETURN again. Now paste each of the following at the prompt and press RETURN after each.

chown root:admin /Applications

chmod 0775 /Applications

chown root:admin /Library

chmod 1775 /Library

chown root:admin /System

chmod 0775 /System

chown root:admin /Users

chmod 0775 /Users

chmod -R -N /Applications

chown -R :admin /Applications/*

Again, my apologies.

Let me know if this solved your problem.

As an approach different from what Kappy has suggested, boot the Lion Recovery Partition (press and hold the ⌘R chord on power-up) and (if the Recovery Partition is working) select and use the Disk Utility tool to try to repair the permissions.

Then just reinstall Lion:

Reinstalling Lion Without Erasing the Drive

Boot to the Recovery HD: Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.

Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the main menu. Repair the Hard Drive and Permissions as follows.

When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list.  In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive.  If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the main menu.

Reinstall Lion: Select Reinstall Lion and click on the Continue button.

You will need an active Internet connection, and I suggest using Ethernet because it is at least three times faster than wireless.

@ Kappy absolutely awesome!!  your solution worked for me and saved me a lot of reinstalling

Never going to share the Imac again that's for sure..

Dear Kappy,

I am new to the support community so I apologize in advance if I am not following the right etiquette. I did the same thing that ericgb: I changed my hd permissions and now I cannot turn on my Imac. (it has Snow Leopard) I tried to follow your steps and I am able to start in single user mode. I typed

mount -uw /

chown root:admin /

chmod 1775 /

reboot

The problem is that when I reboot again I get to the grey screen and the spinning wheel and that's all. Is there any way to perform the second part of your instructions in single user mode?

So far it seems the only thing I can do. I cannot start from the Install disc either. I'll truly appreciate your help, I know now I should have never change the permissions....

If that's exactly what was typed, that's definitely not what Kappy was referencing.  That command has changed the root ownership and protections; the top of the whole file system.

If you can't boot single-user again, then probably the easiest approach is to use a cable and a second Mac, and use Target Disk Mode to access the volume from a different Mac to reset the protections back to their expected settings. 

On a 10.7 test system, the settings are displayed as follows:

ls -aled@ /

The local 10.7 system shows the following (your date and some sizes will vary) for the modes (the drwx...) and the ownership (root:wheel):

drwxr-xr-x  41 root  wheel  1462 Jan 28  2013 /

That's translated into bash as the following two commands to reset the values:

chmod u=rwx,go=rx /

chown root:wheel /

I really wouldn't want the so-called sticky bit set on the root directory; that's the leading 1 bit on that numeric.  if the sticky bit really was enabled, the ls display will show a t in the mode display (the rwx stuff), then the following should clear it:

chmod -t /

You'll need to use sudo on the above commands.

FWIW, that chmod syntax shown above is a different (and newer) version of the (older) numeric syntax Kappy is using in the earlier reply.

FWIW, sudo and single-user are good for clobbering stuff.  Have a backup before using these tools.

FWIW, this is a different question, and best asked in a new thread.

Dear MrHoffman,

Thanks for your help. I was able to use the target mode and I am now backing up some files. I couldn't repair the permissions using the Utility disk. I can perform a repair disk function but the repair permission one is greyed out. (I believe it's because the targeted IMac has Snow Leopard and my laptop is X10.8. I read in some discussions that they should have the same version.

To be honest with you, I don't understand the part of your reply from when you say On a 10.7 test system...

Is any other way I can repair the permissions on target mode other than using the disc utility?

By the way, I am able to turn the computer on single user mode so I could try that approach if I get exact code I need to enter (I've been reading so many different instructions on the matter I am confused)

Or, I guess I should get a computer with Snow Leopard to target my IMac.

Thanks