If a Time Machine backup fails on Mac
Try these suggestions if a Time Machine backup fails to complete on your Mac.
If Time Machine shows a message that an error occurred while preparing the backup, virus scanning software might be preventing backups from completing. Make sure you are running the latest version of the antivirus software, particularly if you have recently updated macOS. Make sure your Time Machine backup disk is excluded from virus scanning. If the issue continues, you may need to temporarily disable the antivirus software or uninstall it.
If Time Machine shows a message that the disk is read-only, there may be an issue with the disk or its file system. Check the backup disk with First Aid from Disk Utility.
If Time Machine can’t create a snapshot, your internal disk may be almost full. Deleting some files can resolve this issue.
If Time Machine shows a message that the disk is full, you can exclude files from the current backup or use a larger disk. The disk should be at least twice the size of the data you’re backing up, since Time Machine keeps the last remaining backup. If there’s more than one backup, restarting your Mac may resolve the issue.
If Time Machine stops, check the Finder to make sure your backup drive is listed. If it’s not, remove any USB hubs, and make sure the backup disk is connected firmly to your Mac.
To check for damaged backups that can prevent your current backup from completing, verify your backup disk.