We repeat it so much you're probably sick of hearing it: back up, back up, back up. It's absolutely vital that you back up the data on your iPhone, iPad and Mac so it's safe in case of malfunction, loss or theft.

But backing up a large-capacity iPhone or iPad creates a bulky file that has to be left somewhere, and those of us who rely on a MacBook or other Mac with a relatively small storage allocation may begrudge this.

You can get around the problem by backing up to iCloud, but iCloud storage is limited too, and you may not want to pay for the extra space. Which leaves us with the option of external storage.

In this tutorial we show how to copy an iTunes backup of an iPhone or iPad to an external hard drive (which is straightforward) and then tell iTunes to back up to that same hard drive in the future (which requires a little more cunning).

Note, very importantly, that you'll need to be able to have your iPhone and the external hard drive plugged into the Mac at the same time, so owners of single-port MacBooks may need to buy a USB hub. For advice on which storage accessories to buy, read Best external storage for iPhones.

Copy the backup to the external hard drive

Let's do the easy bit first.

Get hold of an external drive with plenty of space. Make sure it's formatted correctly and create a folder on the drive with an easy-to-remember name. 'iPhone backup' would be fine.

How to back up iPhone or iPad to external hard drive: Hard drive folder

Make sure iTunes is closed and open Finder. Press Cmd + Shift + G, which will open Go To Folder. Enter the following text:

~/Library/Application Support/MobileSync/

Highlight the folder 'Backup' and hit Cmd + C to copy it.

Now go back into the external folder, open the previously created folder ('iPhone backup') and hit Cmd + V to paste in the backup file. This process could take a while, because you're copying across a large file.

How to back up iPhone or iPad to external hard drive: Copy

Once the process is complete, you can go back and either rename the original file in Finder (which we recommend) or delete it (which is risky, at least until we've checked that the backup has worked).

Tell iTunes to back up to the hard drive in future

Our iPhone backup is now safely stored on the external hard drive; but it would be a pain to have to do this every time a backup is created. Our next step will instruct iTunes to back up to the same external location in future - provided the hard drive remains plugged into the Mac.

Open Terminal. Type the following, then hit Return. (Replace 'ExternalHD' with the name of your external hard drive. And the same applies to 'iPhoneBackup', if you've given a different name to the folder in your external hard drive.)

ln -s /Volumes/ExternalHD/iPhoneBackup/Backup/ ~/Library/Application\ Support/MobileSync/Backup

That should be all you need to do - from now on iTunes will know to save its iPhone backups to the external hard drive and the specified folder within. Note that triggering an iTunes backup when the hard drive isn't plugged in will fail.

Before we rush back and delete the backup file on the Mac, we can check that this has worked properly. Quit Terminal and go back to Finder. Find the folder ~/Library/Application Support/MobileSync/ - and you should now see a shortcut icon in the backup folder with an arrow. (Here's how to find the Library folder.)

How to back up iPhone or iPad to external hard drive: Library folder

Finally, let's test this out properly. Make sure the hard drive and iPhone (or iPad) are both plugged in, and trigger a backup in iTunes. If this works correctly, and the new backup is saved to the external hard drive, you can safely delete the old (renamed) backup file on your Mac and save some space.