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  • Troubleshooting

"Time Machine must create a new backup for you": what you lose, and the space the new one needs

After a failed verification, Time Machine wants to start over. Before you click, know what happens to the old history and whether the drive has room.

5 min read

A dialog says Time Machine completed a verification of your backups. To improve reliability, Time Machine must create a new backup for you. It appears most often with backups on a network drive: a NAS, a shared Mac or an old Time Capsule. Time Machine has decided the existing backup can no longer be trusted.

What starting a new backup means

  • The next backup is a full one. Everything on your Mac is copied again, which takes hours and needs room for the whole Mac on the destination.
  • The old history may not be reachable from Time Machine afterwards. On a network destination the old backup is usually set aside or replaced, so restoring a file from last year may no longer be possible through the normal interface.
  • Both can exist at once. If the old backup is kept, the destination now holds two full copies, and fills up much faster.

Before you click

  1. Check what is in the old backup that you might need. Open Time Machine and browse back for anything you deleted on purpose months ago and might still want. Copy it out now.
  2. Check the destination has room for a full backup. Roughly the used space on your Mac, plus some margin.
  3. If you can, copy the old backup somewhere else first. On a NAS it is a single .sparsebundle file. Copying it keeps the history even if Time Machine stops recognising it.
# how much a full backup will need, roughly
df -h /System/Volumes/Data

# where Time Machine is backing up to
tmutil destinationinfo

Why it keeps happening

Network backups are stored inside a disk image on the NAS. An interrupted write, a NAS that sleeps mid backup, a renamed share or a changed network name can each leave the image in a state Time Machine refuses to trust. If you see this message more than once a year:

  • Back up to a drive plugged into the Mac instead, at least as a second destination. Our guide to backing up to an external drive covers it.
  • Keep the NAS from sleeping during backup hours, and keep its software current.
  • Leave out what does not need versioning, such as caches and virtual machine disks, to make each backup smaller and faster.

If the destination fills up afterwards, see Time Machine backup drive full.

Related: Time Machine .inProgress files.

Common questions

What happens if I let Time Machine create a new backup?

It starts a fresh full backup of your Mac. On network destinations the old backup history is often no longer available through Time Machine, so copy out anything you might need from it first.

Why does Time Machine say it must create a new backup?

Its verification found the existing backup unreliable. It happens mostly on network drives, after interrupted backups, a NAS that slept mid backup, or a renamed share.

How much space does the new Time Machine backup need?

Roughly the space used on your Mac, plus some margin. The first backup is a full copy, and later ones only add changes.

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