SMART error when trying to reinstall El Capitan

Hello all, some background: My 21.5 inch iMac is around 5 years old. Hard drive is 500GB and OS is El Capitan. Recently I tried to update OSX from the App Store and the process stuck at 3 minutes to go for several hours. I switched off and back on again and was able to run Disk Utility Repair which came up with a tick so all seemed ok. There is around 217GB of space left. However when I then went into Safari, the cursor disappeared and the screen went whiteish gray then black and switched off.

This happened several times, ie each restart got the Apple logo and time bar for several minutes before the usual desktop appeared. The last time it happened I got a white screen and a question mark folder.

I searched for a solution (using a MacBook so as not to disturb or otherwise upset the iMac) and decided to try and reboot in safe mode and reinstall the OSX. However when I got to the point to choose which drive to install to, Macintosh HD or Recovery HD, both were shown to have SMART error, and the reinstall process could go no further. This seemed serious and as I was unable to continue or do anything else I then turned off the machine.

I have now turned it back on, not updated any apps or OS, and backed up everything using Time Machine to a Seagate external drive. When it finished backing up the screen went white and had a circle with a diagonal line through it. Turning off then back on again got the desktop with Time Machine showing a full back up had completed.

I would welcome any advice as to the best course of action from here.

Thanks in advance.

A warm welcome to Mac-Forums.

Thank you for your very helpful & detailed post. I'm sorry to say that there is little doubt that your HDD is failing, if not actually failed.

Luckily, it seems that your Time Machine (TM) Backup went okay. At least we presume that for the moment.

I have more experienced & senior colleagues on this Forum who might suggest further options, but to me, it's a trip to Apple to get them run their diagnostics (free) and replace your your HDD if failure is confirmed (not free).

If you have the technical skills, you might try replacing the HDD yourself, but it is very tricky on the iMacs in general & your one in particular.

I'm really sorry this has happened to you.

Ian

I would welcome any advice as to the best course of action from here.

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As Ian stated above, your hard drive has failed and needs to be replaced. Since your iMac is out of warranty, you have several choices: You can take it to Apple for repair or do it yourself. The difficulty of getting into an iMac and replacing the hard drive (without breaking anything else) is not the only problem. Your hard drive will have to be replaced with an exact Apple replacement or the fans in your iMac will roar like a 747 on takeoff. There are ways around that which you can Google and read about. You might want to take a look at the hard drive replacement procedure at www.ifixit.com.

SMART never lies alas. Always pays to run Verify over a drive prior to updating.