How to Give a drive a health check >
1 Open Disk Utility
Disk Utility lives in Utilities within the Applications folder. As usual, the quickest way to reach it is to press ç+[ spacebar] to open Spotlight and start typing its name. Its sidebar shows all the drives connected to your Mac, inside and out.
2 View devices/volumes
By default, you see ‘volumes’, logical spaces for storing files (like Macintosh HD). If you want to see which physical drives they’re on and those drives’ structures, pick View > Show All Devices (or press ç+2; ç+1 shows volumes).
3 Your startup disk
Whether your startup disk is an internal or external drive, selecting it on the left dims out the Erase, Restore and Unmount options. You can run First Aid, but repairs may need you to use Disk Utility in macOS Recovery (see page 25).
4 Run First Aid
On other drives, First Aid just asks for confirmation before starting its checks, which can take a minute or two. It’ll automatically repair any minor issues that it finds and warn you if any problems are found that it can’t fix.
5 Disk permissions
Until OS X Yosemite, there was an option to verify disk permissions, which could clear up problems such as changes to macOS failing to ‘take’ or apps not opening. These don’t occur now; macOS is meant to deal with them as it goes.
6 Time to erase
If First Aid can’t repair errors on your startup disk, see page 25. For any other drive, select an empty drive and use Restore to copy contents to it. Erase the problematic drive and try using it, but keep an eye on it for failing health.