"The backup disk image could not be created": Time Machine on a network drive, and the causes in order
Network Time Machine backups live inside a disk image the Mac creates on the share. When it cannot, it is usually space, permissions or the share's settings.
You set up Time Machine to back up to a NAS or a shared folder on another Mac, and the first backup fails with The backup disk image could not be created, sometimes followed by a path ending in .sparsebundle. For network destinations, Time Machine stores everything inside a disk image it creates on the share. It could not create it.
Check these in order
| Cause | How to check |
|---|---|
| Not enough free space on the share | The share needs room for roughly a full copy of your Mac |
| A quota on the share or your account | NAS settings for shared folder or user quotas |
| The account cannot write there | Try copying a file to the share from Finder |
| The share is not set up for Time Machine | Most NAS systems have an explicit Time Machine option per share |
| An old, half created image is in the way | Look for a .sparsebundle named after your Mac on the share |
The usual fix
- On the NAS, enable the Time Machine option for the shared folder and give your account read and write access.
- Check free space and any quota. Leave headroom beyond the size of your Mac.
- If a partial
.sparsebundlefrom a failed attempt is on the share, remove it, so Time Machine starts clean. - In System Settings, General, Time Machine, remove the destination and add it again.
Size it before you start
df -h /System/Volumes/Data # used space is roughly the first backup
tmutil destinationinfoLeaving out folders that do not need backing up, such as virtual machine disks and caches, makes the first backup smaller and faster. Our guide to Time Machine taking up space lists what is worth excluding.
If backups started and later the Mac asks to start over, see Time Machine must create a new backup.
Common questions
What does The backup disk image could not be created mean?
Time Machine could not create the disk image it uses to store backups on a network share. The share is usually short of space, has a quota, is not writable by your account, or is not set up for Time Machine.
How much space does a network Time Machine backup need?
Roughly the used space on your Mac for the first backup, plus headroom for later changes. A quota on the share can block it even when the disk has space.
Should I delete an old sparsebundle on my NAS?
Only a partial one from a failed first attempt, so Time Machine can start clean. An existing backup holds your history, so copy it elsewhere before removing it.