Time Machine backup failed or couldn't be completed
Quick answer
Connect the backup drive straight to the Mac, with no USB hub, and run First Aid on it in Disk Utility. If the disk is full, exclude big items in Time Machine Options or use a bigger drive.
Why it happens
Time Machine stops when it can't write safely to the backup drive. The usual reasons are errors on the drive, a drive that's nearly full, or one that was unplugged or lost power in the middle of a backup. Hubs and long cables make dropouts more likely.
How to fix it
Plug the backup drive directly into a port on your Mac. Skip hubs, docks and extension cables. If the drive has its own power adapter, make sure it's switched on.
Open Disk Utility: press Command-Space, type Disk Utility and press Return. Choose View > Show All Devices so you can see every drive.
Select your backup drive's volume in the sidebar, click First Aid, then Run. Wait for it to finish; it can take a while on a big drive.
Disk Utility›First Aid
If the drive is nearly full, open System Settings > General > Time Machine and click Options… to see what's left out. Use the + button to exclude big folders you don't need backed up, like Downloads.
General›Time Machine›Options…
Start a backup again: click the Time Machine icon in the menu bar and choose Back Up Now. No icon? Turn it on in Menu Bar settings, under Menu Bar Controls.
Menu Bar›Menu Bar Controls›Time Machine
Still not fixed?
If Time Machine says it must erase your existing backup history and start a new backup, click Remind Me Tomorrow. Restore any older file you need first, then let it start over with Erase Backup History.
If it says the identity of the backup disk has changed, reselect the disk in General > Time Machine. Give other drives a different name from your backup drive.
General›Time Machine
Antivirus or cleaning apps can block Time Machine. Set them to skip the backup drive, then try again.
If First Aid says it can't repair the drive, or backups keep failing with a different cable, the drive is probably failing. That's a hardware problem: back up to a new drive and keep the old one only for its history.
If a brand new drive also fails, contact Apple Support. The problem may be with your Mac's ports or its own disk.
Questions people ask
Why does my Time Machine backup keep failing?
Usually the drive has errors, is nearly full, or disconnects during the backup. Plug it in directly and run First Aid in Disk Utility.
Does Time Machine delete old backups when the disk is full?
Yes, it removes the oldest ones by itself. If it still can't fit a backup, exclude big folders in Options or use a bigger drive.
Can I use a USB hub for my Time Machine drive?
It often works, but hubs are a common cause of failed backups. If backups fail, plug the drive straight into the Mac.
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