Mac won't won't boot up after trying to update latest OS [duplicate]

Out of the blue, my iMac (late 2010, running El Capitan) started refusing to boot. The progress bar would take forever to reach 100%, and then it would just stop there once it reached it; all the while the fan running loudly.

I am able to boot into Recovery Mode, and I can even run Disk Utility (it doesn't detect any problems.) However, when I attempt to reinstall the OS, it tells me:

This disk has S.M.A.R.T. errors.

This disk has a hardware problem that can't be repaired. Back up as much of the data as possible and replace the disk.

I have an outdated Time Machine backup, made before I upgraded from Mountain Lion to El Capitan. Most of the data in the backup is the same, but there are a few GBs of new files I'd like to backup as well.

Since Disk Utility tells me everything is okay, am I able to somehow mount an external, and backup files? Possibly through Terminal? And If I were to use the drive with the old Time Machine backup on it, would I have to delete the backup before writing new data to the drive?

I'm not very computer savvy, but I can manage my way through Terminal with a lot of instruction. So please, try to answer in terms I will understand. Thanks.

Whatever you do, leave the faulty disk alone until you know what you are going to do. The risk that the disk breaks entirely any moment now is very high.

This answer assumes you don't have FileVault enabled.

1. Get an external backup disk that has enough capacity and connect it.
2. Press cmd+alt+R before the boot screen appears to enter internet recovery mode. This loads the recovery mode from the internet, which will be very slow. The advantage is that there is no stress on the faulty disk.

Method A – If you want to save individual files:

1. Go to the menu bar and select Terminal.

2. Copy the files you want to rescue using cp. For instance, if you have a folder rescueme on your desktop, you can use the following command:

   cp -r /Volumes/mydisk/Users/myusername/Desktop/rescueme /Volumes/mybackupdisk/ This will copy all files and subfolders of rescueme onto your backup drive called mybackupdisk.

Method B – If you want to make a full backup:

1. Enter the Disk Utility tool
2. Select the Disk that you want to back up
3. In the menu bar select File > New Image > Image from “mydisk”
4. Select your options. E.g. you may want to compress the image to save some space on the backup disk.
5. Press Save and wait.

It is likely that some parts of your disk are not readable anymore. Therefore the method B may not work and you should first try to save the most important files manually (i.e. method A).

Method C - if you have another Mac:

1. Connect the two computers, e.g. with a Thunderbolt cable.
2. On your broken iMac, press T at the boot screen to enter target disk mode.
3. Now you can use the Finder to browse the broken disk and copy files.

You can use terminal to selectively backup files from your iMac, especially the files and folders in your Home folder. The cp command would be just the right tool for the job.

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.

Because it is a simple copy and paste command, you don't have to delete the backup in your Time Machine disk. Just make sure that disk has sufficient storage space for the things you want to copy.

I personally used and found the cp command to be the fastest and most straightforward when you just want to backup the important folders in my Mac.