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A couple of days ago my iMac (Late 2009) stopped turning on. It starts booting and the progress bar under the Apple logo goes to just under halfway before it turns off. Booting into recovery mode and trying to reinstall El Capitan doesn't work because it says the hard disk is locked. I tried running the chflags 0 and chmod to unlock it that are suggested around here from both single user mode and the recovery terminal but to no avail.

The files on the hard drive are all still there, so I tried to use cp to copy some stuff from there to USB disk, but it consistently crashes midway through the copy and goes to the "your computer has had a problem" screen. I've also tried zapping the PRAM.

Running first aid on Disk Utility from recovery mode says "file system verify or repair failed". I also tried booting in verbose mode and noticed it crashes after running fsck_hfs and fails, which I assume is what disk utility is doing behind the scenes.

Any advice on how to backup and then do a clean install or reinstall from recovery or just fix it would be very appreciated!

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    Most likely your hard drive is failing. What you should do is replace it with a new drive (SSD is best) and try to recover your data by attaching your old drive via USB – Allan Jan 4 '17 at 15:33
  • If it is failing, wouldn't that mean the files are inaccessible? – sc8ing Jan 4 '17 at 15:54
  • Not necessarily; you may be able to access some files. The fact you have so many errors related to reading your drive is an indication of a failing drive. Issue the command diskutil info disk0 | grep -i smart and post the results. Do you have a Time Machine back up? – Allan Jan 4 '17 at 16:02
  • Also, to find out what specific model you have issue the command system_profiler SPHardwareDataType | grep Identifier. – Allan Jan 4 '17 at 16:08
  • Oh, I see. It says "SMART Status: Verified" for the diskutil command and "Model Identifier: iMac10,1" for system_profiler. And no, no time machine backup. – sc8ing Jan 4 '17 at 16:26
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The symptoms you describe are indicative of a disk drive corruption. It could be physical (hardware) or just a file system corruption.

Recovery will require booting from a different drive so you can copy files over. You can boot from an external USB that has a clean install of OS X (El Capitan would be the latest as your system is not capable of running Sierra) or replace the internal drive with an SSD and attempt recovery via USB.

Personally, I prefer the latter.

The full repair guide is available on ifixit.com, but here are the basic steps you need to go through

You will need the following:

  • New SSD Drive. I reccomend the Samsung 850 EVO
  • USB to 3.5" SATA adapter. This is so you can access the drive for recovery
  • El Captian Installation Media
  • Suction Cups (for glass removal)
  • Torx T10 screwdriver
  • plastic spudger for connector removal
  • 2.5" to 3.5" kit so the new drive will fit
  • External HDD for Time Machine (Optional, but highly, highly recommended)

Remove the glass. The good news is that the glass panel is held in with magnets which you can remove with suction cups and the LCD itself is held in with 10 Torx screws.

imac display removal enter image description here

Replace the drive. Once you have the panel removed, you will get access to the 3.5" Hard Drive (see pic below)

imac panel off

When you replace the drive, ensure that you replace the temperature sensor, otherwise you will end up with a fan that spins uncontrollably, and erroneous temperature readings.

Just reverse the steps to re-assemble your iMac.

Clean Install OS X. Using the boot media, do a clean installation of OS X. Then reinstall your apps.

Being recovery. Using the USB to SATA adapter, hook up your old hard drive to your iMac via USB. You should be able to mount it and cp (copy) your data off. Once done, you can discard your drive (it's quite old and I wouldn't trust it for data storage anymore)

Set up Time Machine. I can't stress this enough... having a backup besides (or in addition to) iCloud is extremely important. Right now, you are finding out first hand how valuable a Time Machine backup can be.

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That was the exact problem that I faced! Most likely it is a file system corruption that can only be fix via a clean install of the OS.

1. Backup all important files and folders

You can use terminal to selectively backup files from your mac, especially the files and folders in your Home folder.

cp -r -v /Volumes/Macintosh HD/Users/user-name/ /Volumes/Backup

This is the command, where /Volumes/Macintosh HD/Users/user-name is the source and /Volumes/Backup is the destination. The parameter -r means to copy recursively including the files inside each subdirectory. -v is to enable verbose mode, so that you can see all the details while the command is running. You would need to change the hard disk and folders name according to those inside your iMac and external drive.

There are other methods to recover your files, but I personally prefer to backup via the cp command.

2. Install The OS

Before you jump to install, make sure all your files are properly backup. Check by opening the files to make sure they are not damaged.

You can boot into recovery mode by holding down on the CMD + R keys until you see the Apple logo. Choose the Reinstall Mac OS option. You'll need to connect to the internet to download and restore the system

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