Normally when you plug in an external hard drive to your Mac's USB port you will see it mount on the desktop. You can also see it in the Finder in the left-hand column under Devices.
Sometimes though the drive fails to mount - usually when you desperately need to access some data stored on it. Should this happen here are the steps to follow to hopefully gain access to the drive.
We have this separate article that explains how to recover data if it turns out that the drive is corrupt.
This article assumes you have an external drive that connects to your Mac via the USB or Firewire port. If you have a NAS drive that connects over the network read this article about connecting to a NAS drive.
- Make sure your Mac is set to show mounted drives on the desktop. Go to Finder > Preferences > General and make sure that there is a tick beside External Drives.
- Check the drive is properly plugged in. It sounds obvious but one of the main reasons why drives fail to mount is if the drive isn’t receiving enough power. If the drive is powered via a USB cable you need to check that adequate power is being delivered to the drive. Older Macs may require a USB power cable, a cable that splits into two USB connectors that need to both be plugged into your Mac, in order to deliver enough power to the drive. Similarly, make sure that the drive doesn't have an external power supply it should be using.
- On the subject of USB cables, make sure that it’s not at fault. Try using a different USB cable with the external drive to see if that fixes the problem. (Equally, if you are using a USB port on a hub check that’s not what’s causing the problem).
- Also check that the USB port on your Mac isn’t the problem. Try plugging into a different USB port. Or if you only have the one, plug another device in and see if that works ok.
- Try another Mac to see if you can access the drive there.
- Try a PC - perhaps the drive is formatted for PCs and can’t be read by your Mac.
- Open Apple’s Disk Utility program to see if you can get the Mac to see the drive there. Find Disk Utility by opening Spotlight (cmd+space-bar) and start typing Disk Utility, press enter to open the program. Look in the list on the left to see if the hard drive appears there.
- If you can see the hard drive in Disk Utility check underneath it for a volume. If it is there click on it and select Mount. If your Mac has already mounted the drive the option Unmount will be displayed instead.
- If there is no volume listed your Mac is not able to access the drive. The Mount option will be greyed out.
- The only choices here are First Air, Erase and Restore. First Aid will check the disk for errors and then repair the disk if necessary. Restore allows you to erase the contents of the drive and replace that with volume from another drive. And Erase deletes all the data stored on the drive. This cannot be undone. If you need the data on the drive do not erase!
- You could try running First Aid. Click the "First Aid" tab and select Run. If after running First Aid the Mac finds errors you could fix you may see the option to ”Repair Disk.”
- If you Mac is unable to repair the disk if is likely that the drive is either formatted using a file system that the Mac cannot read, or it is well and truly broken. If it’s the latter we suggest you follow this tutorial about recovering data from a damaged disk.
- A bit of background on file formats: Windows PCs use NTFS file format, while Mac computers, prior to Sierra, used the HFS+ file format. Just to complicate matters, Apple’s now introduced a new file system called Apple File System (APFS). However, it is possible to format a drive so it can be ready by Window and Mac computers - those drives need to be formatted using exFAT or the older FAT32. If you think the hard drive has been formatted using a different file system (i.e. on a PC) and you need to access the data on the drive, connect your drive to a computer that does recognise it and copy the data before moving on to the next step.
- If you don’t require the data on the hard drive, click Erase and your Mac will reformat the hard drive. For details about how to format the drive so you can use it with Macs and PCs read this.
Make sure that your external drive doesn’t become damaged in the future by always unmounting the disk properly after using it. Don’t just unplug the USB cable. To unmount your drive you can right-click (control-click) on the icon on the Desktop or in the Finder and choose Eject. Apparently most drive issues are caused when the disk is removed without ejecting it properly.