Time machine is a great way to keep your Mac backed up, it’s also a simple way to move all your files and software to another Mac, but as the contents of your Mac increases you may find that your Time Machine can’t complete the backup due to lack of space.

Here we will explain what to do if your Time Machine backup failed because there isn't enough space on your hard drive. Find out what to do if your Time Machine back up is full, including which Time Machine backups you can delete, and how to reduce the size of your Time Machine backup.

If you want more information on how to use Time Machine, we have a complete guide to Time Machine here.

What to do if your Time Machine backup failed

From time to time you may see the following message when Time Machine attempts to back up your Mac:

Time Machine could not complete the backup
This backup it too large for the backup disk
Backup requires X GB but only Y GB are available

If you see that message you may assume that you need to get a new hard drive to use for backups, but that’s not necessary. It is possible to delete older backups from your Time Machine backup and it’s possible to change the information you are backing up so that it doesn’t require as much space.

Before you start - check that you haven’t copied any other information to the backup drive that you can do without. Especially if it’s an additional back up of your photo library, which is backed up by Time Machine anyway. If there is any data on the backup drive that is also in your Time Machine backup you don’t need to duplicate it.

Another thing to check is whether there is more than one Mac using the drive as a backup drive, if for example your Mac is backing up to a shared NAS drive (network attached storage).

How to delete old Time Machine backups

Time Machine should automatically delete the oldest backups when your disk becomes full, however, there may be circumstances when you need to delete old backups yourself, perhaps if you are attempting to back up a lot of information (because you hadn’t made a Time Machine back up for a while, for example).

Even with Time Machine automatically deleting older backups, as the amount of files increases on your Mac, eventually your back up disk will become full.

If you do run out of space, and you don’t have a new storage device handy with which to start a new backup, deleting some files from the backup is the place to start.

  1. Connect the drive to your Mac.
  2. The drive should mount on your Desktop.
  3. Click on the drive and view the contents in the Finder.
  4. Click on the Backups.backups folder.
  5. Scroll through the various folders that will appear in date order, oldest first.
  6. Find a folder you want to delete - you could probably delete one of the older folders, after all it’s unlikely you need Time Machine to go back that far.
  7. Right-click/control-click on it and choose Move to Trash.
  8. You will see a warning that ‘You can’t undo this action. Do you want to move the backup to the Trash anyway?”
  9. Click on Continue.
  10. Enter your password if prompted.
  11. To ensure it is deleted from the hard drive, go to the Trash and right click and choose Delete Immediately.
  12. Confirm that you want to delete and enter your password again.
    You may see a couple of warnings that you can’t delete something because it’s in use. Just click on skip and continue deleting.

You can also delete a backup following these instructions:

  1. Click on your backup drive so that the contents opens in the Finder.
  2. Now, click on the Time Machine icon in the menu bar.
  3. Click on Enter Time Machine.
  4. Click on the backup you want to delete.
  5. Click on the gear icon and click on Delete All Backups of “folder name”.
  6. You may see the warning, Are you sure you want to permanently delete all backups of the selected item, click OK.
  7. Enter admin password if required.

How to delete a large file from a backup

Deleting a whole backup may save you some space, but what if there is a particular file or folder that’s is backed up unnecessarily? Here’s how to delete that:

  1. Open the Finder and locate the item you wish to delete from the back up, for example, a movie download or a folder with images in it that you don’t need to back up.
  2. Click on the Time Machine icon in the menu bar.
  3. Click on Enter Time Machine.
  4. Navigate to the point in time that you want to delete.
  5. Click on the gear icon and choose ‘Delete Backup”.
  6. Enter admin password if required.

How to exclude things from the back up

If you want to keep your backups on the slim side, it’s possible to tell Time Machine what to back up and what to ignore.

If you want to exclude items from your backup, follow these steps:

  1. Click the Time Machine icon in your menu.
  2. Click on Open Time Machine Preferences (you can also access Time Machine from within System Preferences).
  3. Click on Options.
  4. You’ll see a window showing the items already excluded from the backup.
  5. Click on the + and navigate to file or folder you want to leave out of the backup.

Tips to keep your backup small

If you are struggling with the size of your backup, here are some tips.

  • If you have a large music library don’t back it up, instead sign up for iTunes Match. Then it will be backed up in Apple's iCloud, and you will be able to access it from any account.
  • If you have a large photo library, sign up for iCloud Photo Library, that way the photos on your Mac will be backed up to the cloud. You just need to make sure you have adequate space in iCloud, which may involve a monthly subscription charge. We have all the details for how much iCloud storage space costs here.
  • If you are running virtualisation software, such as Parallels or VMware you may have large disk images related to another operating system on your Mac. Time Machine might back up the whole disk image even if you have creased only a few files in the other OS. Tell Time Machine to exclude those files from the backup.
  • Are you backing up a large email folder? If you use Outlook, for example, it’s email database could be very large and Time Machine might be backing the whole thing up. If you are syncing Gmail for example, you will have a backup on the Internet, so you don’t need it duplicated here.