Normally when you plug in an external hard drive to your Mac’s USB port you will see it appear on the desktop (aka mount on the desktop). You can also see it in the Finder in the left column under Locations (or Devices in older versions of macOS). If the drive is not showing up on your Mac chances are it has not been formatted correctly, has been corrupted, or is faulty. Alternatively, you may have set a password for the drive that you need to enter before it will connect.
We show you how to work out whether the reason the external drive is not mounting on your Mac up is due to a problem with the drive itself, the cable or the port, and how to resolve the problem and access data on your drive.
This article assumes you have an external drive that should connect to your Mac via the USB-A, USB-C, or a Thunderbolt port. If you have a NAS drive that connects over the network then you should read our article about connecting to a NAS drive.
Read more at Macworld.com