iMac doesn't start up

A few questions:

- Do you have an external hard drive with backup of the internal hard drive? If not it might be a good idea to try and get one incase you have to erase the internal hard drive...a good source is OWC, I have three of their Mercury Pro Elite 2 TB drives.

- Have you tried starting up in Safe Mode - startup holding the shift key, if you get a good startup then there may be issues with applications of extensions that Disk utility cannot resolve.

- If there is physical damage to the hard drive, the only options will be a new hard drive or an erase and install of the operating system, and all applications, so that the bad sectors can be avoided. This step should not be done until you have a backup of the drive.

Another idea: You might be able to fix this by running fsck in Single User mode. You would repeat this until possibly it is able finally to make the repairs. It's worth a try anyway.

See

Use fsck if necessary

http://support.apple.com/kb/TS1417

I would agree with WZZZ, if you can safe boot then at the command prompt type /sbin/fsck -fy noting that there is a space between fsck and -fy...read the messages you get and when it stops finding errors, may take a couple times running fsck, then exit and reboot.

Disk Utility runs fsck behind its nice graphic interface, which is why the kb article says it really isn't necessary, but with problems like you are having it is a way of getting around the need for a full graphical system boot.

I would agree with WZZZ, if you can safe boot....

Ralph, not a typo? Don't you mean single user to run fsck? If the OP can Safe Boot, it would mean the directory has been repaired.

Yes, I definitely need another cup of coffee this morning 😟

Single User Modestatup explanation. If you get a good startup in safe mode, you should be able to copy material to your external drive. But if the damaged file systems are the ones that are causing problems starting up, well they will still be a problem. Anyway, try the safe mode startup and see what happens. If all goes well try to copy your important material. Then do the single user startup and run fsck to see if that can clean things up for you.

If the Mac starts up successfully in Safe Mode, that means the drive is repaired. Give it much longer to boot in Safe Mode than with a regular boot. In that case, you can make a new backup. But in what form is the backup, Time Machine, a bootable clone or just files and folders copied over?

If it won't Safe Boot, that means you go onto booting into single user mode. Hold CMD-S keys down at startup, then follow the directions for fsck in the article I linked. single user will present you with a special screen that has only text. You may need to run fsck a number of times before the drive is repaired, if it's going to be repaired at all this way.

If it doesn't start in Safe Mode (not single user) that doesn't necessarily mean a hardware failure.

Keep running it, but I/O error doesn't sound good. Sounds like the drive has failed or may be failing.

Message was edited by: WZZZ

If you want to preserve the data on the boot drive, you must try to back up now, before you do anything else. It may or may not be possible. If you don't care about the data, you can skip this step.

There are several ways to back up a Mac that is unable to fully boot. You need an external hard drive to hold the backup data.

1. Boot into Recovery (command-R at startup) or from a local Time Machine backup volume (option key at startup.) Launch Disk Utility and follow the instructions in the support article linked below, under “Instructions for backing up to an external hard disk via Disk Utility.”

How to back up and restore your files

2. If you have access to a working Mac, and both it and the non-working Mac have FireWire or Thunderbolt ports, boot the non-working Mac in target disk mode by holding down the key combination command-T at the startup chime. Connect the two Macs with a FireWire or Thunderbolt cable. The internal drive of the machine running in target mode will mount as an external drive on the other machine. Copy the data to another drive. This technique won't work with USB, Ethernet, Wi-Fi, or Bluetooth.

How to use and troubleshoot FireWire target disk mode

3. If the internal drive of the non-working Mac is user-replaceable, remove it and mount it in an external enclosure or drive dock. Use another Mac to copy the data.

As WZZZ said, run fsck over until it comes back without error messages...if it doesn't, is the external backup drive you have a bootable drive? Time Machine is not bootable so you need a clone copy of the operating system on the drive to boot.

If you start up from the recovery HD, one of the options is to do an erase and install which will sanitize the drive, and then restore from the TM backup. If you can start from an external drive, a clone of the internal, you will be able to see the internal hard drive and open it. You can copy material, as long as it isn't eamaged, to the external drive.

If the drive is failing it may be determined to fail sooner or later, so this may not be a long term solution, and I'm not sure I would trust a drive that has been "reclaimed" this way. You don't want to do a simple erase and install. What you can try is first to reformat the drive from Disk Utility. Then, select Secure Erase>Options>One Pass. Then restore from TM.

On formatting:

http://pondini.org/OSX/DU1.html

Secure Erase one pass

http://pondini.org/OSX/DU2.html

Then see if you can restore from your TM backup.

http://pondini.org/TM/14.html

Message was edited by: WZZZ

Note: I haven't done this in a while, so don't remember if it's zero first then reformat or vice versa, but it wlll be clear when you do it.