It's all very well telling people to back up. But human nature means that searching for backup advice is something that usually happens after the worst has already happened. So here's what you need to know about crashing without a backup and how to delete corrupted files on an external drive.
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.
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.
(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.
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.
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