This is also relevant if you have accidentally deleted a file. Deleted files can be retrieved too, but only if they haven't been overwritten. Once the part of the disk with the data is overwritten, nothing can get it back.
This is part of a series of articles about backing up your Mac, read more here:
How to back up your Mac: Three types of backup all Mac users should be using Best cloud based, live backup options Best bootable back ups for Mac Best remote backup options
How to solve a hard disk or SSD problem on Mac
If your drive seems to be having problems, but you are still up and running, the first thing to do is launch Disk Utilities. You'll find this in the Utilities folder, inside your Apps folder or by searching for it in Spotlight Search. Click on the disk and then click either Verify Disk, Repair Disk, or First Aid. This is a good first step and if it's an easily fixed problem, then it could resolve it quickly.
It's also a good idea to fix Disk Permissions too. In fact doing this regularly is advisable, as incorrect disk permissions can cause all sorts of issues. It's the first port of call for any, "My Mac is being weird", issues.
If your drive isn't a disk but an SSD (Solid State Drive) you may find the problem is more serious. SSDs are fundamentally different from HDDs, and although they seem to act the same, fixing them is a whole different matter. If fixing permissions doesn't help, you can still follow the same process to attempt to save the data. But the chances of recovery are far worse. When you delete data, SSDs actively reset their memory to be blank, while HDDs simply ignore any data in a block and only overwrite it if there is new data to store. Read: SSD versus Hard Drive.
(Not) Turning it on and off again to solve a disk problem
It's become something of a clich, but turning it off and on again, is often a good way to reset any gremlins in your system. So it's often the first bit of advice you'll hear. However when dealing with dying disks, they may not survive the experience.
If you tried to restart and the drive didn't survive the experience, you may find yourself in an even stickier situation. Now that modern Macs no longer ship with optical disk drives, you may not have an easy option to boot from another disk. Read: How to make a bootable OS X Yosemite install drive
However, as long as the drive failure isn't too terminal, a restart should reveal Recovery HD. This is actually a partition on the drive that is invisible in most circumstances, and only shows up when you most need it. Booting from this partition doesn't do anything to the main drive, so there's no need to worry about overwriting documents.
Read our round up of the best storage devices available now: Best storage options for Mac
What to do if your Mac has started from Recovery HD
If you have started from Recovery HD, then with luck your Mac will see the offending drive. At this point, run the Disk Utility from the Recovery HD and see if that helps. If you have any disk management tools, like TechTool Pro, Disk Warrior or Drive Genius, this is a good time to try them out. They all claim to be more effective than Disk Utility, but how well they do is very dependent on whatever is at the root of the problem. However, if you attempt to use these tools on an SSD, you'll most likely find they aren't quite so helpful. The way SSDs work is very different from HDDs that most traditional tools struggle to help at all.
One thing to bear in mind is that if you do manage to boot from Recovery HD, you'll need another drive to recover files to. If any of the disk utilities manage to fix your drive enough to let you boot from it again, be very careful how you use it. You should plan to get an external drive plugged in as soon as you can, so that you can secure your data there. If you have lost any data, it is very important to avoid using the drive, as anything that writes files to the drive, may be writing over your lost files. Which would render them unrecoverable.
So if you manage to resurrect your sickly drive, quit all your software, including email, to avoid overwriting any list files. Once you have an external drive, you can then use that as your boot drive, and recover any lost files to there. Read next: How to reinstall Mac OS X using Recovery mode.
What to do if the Recovery HD is missing on Mac
If you are using an old Mac running an OS older than Lion, then there will be no Recovery HD option. Lion has been around since 2011, so anything newer than that should have the Recovery HD partition available if the drive is showing any signs of life. If you have an older Mac, you will need to boot from an external drive either optical or HDD, that has OS X on it.
If your machine is running Lion or newer, and the Recovery HD isn't showing, then it does suggest the drive problems are terminal. But you may as well try to boot from an external drive running OS X.
How to boot from another drive on Mac
If you are able to get either a DVD drive or an external drive with OS X already installed, you'll be able to boot from another drive.
Restart your Mac and hold down the Alt/Option key. When your machine gets to the point where it looks for an operating system to start from, it will give you the option to choose any suitable drive to start from.
This puts you in a pretty good position to proceed, as you will no longer be using the faulty disk, and you won't be in danger of overwriting any lost data. You can try running Disk Utilities at this point. But it's more likely that the problem will warrant using a data recovery App to retrieve any data you can. Read is it worth buying an Apple SuperDrive
Also read: How to chose best storage: SSD or Hard Drive.
How to delete corrupted files on external Mac drive
Having an external drive is useful for backups and having the ability to install OS X separately, however sometimes these very drives are the problem-child.
There might be corrupt files within the drive, or you might have a completely corrupt external drive, in either case, we presume your primary hard disk on your Mac is operational, meaning you'll be able to launch OS X and run Disk Utilities. It might seem like the obvious choice to go to, but it's by far the best method to solve corrupt files on a hard disk that wants to run alongside OS X.
From Disk Utilities, we suggest running First Aid. If that still doesn't help, you can manually go into the Finder and delete the individual files you think are causing you problems.
If on the other hand, the whole drive is corrupt and you've tried every last resort to extract data from it and failed, then you last-resort option might be to fully format the drive and hope you can at least use it after the deletion process has passed. Before proceeding, we would urge you to try using data recovery software or even visiting a computer specialist to extract the data off your drive.
We have an extended guide on how to format a storage drive for Mac - follow those steps and you should hopefully have a functioning drive.
Use data recovery software on Mac
If you have managed to start your machine from either the Recovery HD, or an external drive, the volume you are trying to recover may not show up. Data recovery software can still work with that drive, so long as the OS can still talk to it. If the data is still on the drive, you still have a reasonable chance of recovering it.
There is one exception though, and that is if you were using an SSD. Because if the nature of SSDs, it is often impossible to retrieve data from them, no matter which tool you are using. This is because, unlike HDDs, SSDs would be very slow indeed if they had to save data to a block, or cell, that already has data on it. HDDs can merrily over write data over and over again. But SSDs must first erase that data to be able to quickly record new data. So SSDs have various strategies to manage this. OS X uses the TRIM system, which zeroes out the cells holding data is deleted, as part of is disk management. Other SSDs will always use a similar system, because if they needed erase and record at the same time, they would end up being slower than HDDs.
The end result of that is that if you're using an SSD and it fails, the chances of any data recovery software retrieving that data is slim to none. A sobering thought.
Read: Best SSD to upgrade Mac, internal SSD options and Best SSD for Mac
Choosing the right data recovery software for Mac
The annoying thing about data recovery software is that you only really use it if you have been caught without a backup. The developers have you at their mercy, and if you have lost precious data they could charge whatever they can get away with. But it is a highly competitive market too so the pricing structure has evolved at a sort of equilibrium. Pretty much all data recovery software costs $99 or the UK £ equivalent of that.
The good news though, is that developers don't want to you spend the $99 to then comeback and complain that it didn't work. Thus a vast majority offer a free version that will scan your disk and see what is salvageable. You can then decide of the fragments of your dead drive are worth $99 to retrieve.
All work in a similar way. If the directory telling where files were originally stored is saveable, the rest is easy. But if that isn't the case, the software will scan the data for familiar patterns that would denote a file type. Once they find as file type, there's a good change that the following data will be the file. Highly fragmented hard drives might mean not all files can be retrieved, but given enough time, most will get there in the end.
It can be a time consuming process to piece the bits of files back together. But with patience, most files that haven't been overwritten should in theory be saveable. If a hard drive is still spinning, and the head is still scanning, there is still hope. Sickly drives can take days to fully scan, and the higher capacity they are, the longer it will take. But if it's your wedding photos, or perhaps your Bitcoin vault that is missing, you can afford to wait.
Conclusion
If you don't need data recovery right now, then do yourself a huge favour and make sure your backup strategy is up and running and fool proof. A little pain now will save you a world of hurt in the future. If you have already lost some data, then the process outlined here will help. But without wishing to pour salt in the wound, now would be a really good time to take a look at your back-up strategy. You can find a very comprehensive backup strategy here: How to back up your Mac.
Also read:
Best cloud based, live backup options Best bootable back ups for Mac Best remote backup options How to defrag and speed up a Mac