macOS Error 100008 — Time Machine Backup Failed Fix

What Does This Error Mean?

macOS error 100008 appears when Time Machine cannot complete a backup. The specific message is usually 'Backup failed with error: 100008.' This means Time Machine lost contact with the backup destination — either the backup drive, a Time Capsule, or a NAS device — during the backup process. The backup was interrupted before it could finish.

Affected Models

MacBook Air
MacBook Pro
iMac
Mac Mini
Mac Pro
Mac Studio

Common Causes

The backup drive was disconnected or went to sleep during the backup
A Time Capsule or NAS backup destination went offline or became unreachable over the network
The backup drive is running low on space and could not complete the backup
The macOS file system permissions on the backup destination are incorrect
A corrupted backup catalog on the destination drive is preventing new backups

How to Fix It

Check that the backup drive is connected and awake. If using an external drive, make sure it is plugged in and powered on. Go to System Settings > General > Time Machine and verify the backup disk is listed and shown as available.

The most common cause of this error is simply the drive going to sleep or being accidentally unplugged. Reconnect it and try a manual backup.
Start a manual backup. Go to System Settings > General > Time Machine and click 'Back Up Now.' Watch to see if the backup starts and how far it gets before failing.

A manual backup gives you more visibility into when and why it fails. Check the backup log by holding Option and clicking the Time Machine icon in the menu bar.
Check available space on the backup drive. In Finder, right-click the backup drive and choose Get Info. Ensure it has enough free space to accommodate your new backup.

Time Machine needs roughly the same amount of free space as the data being backed up, plus room to maintain backup history.
Remove and re-add the backup disk. Go to System Settings > General > Time Machine, click the minus button to remove your current backup disk, then add it again.

Re-adding the disk resets the Time Machine connection and clears configuration errors that cause this error.
If the backup catalog is corrupted, delete it and start fresh. In Terminal: sudo tmutil deletelocalsnapshots / — then remove the backup disk in Time Machine settings, erase the backup drive using Disk Utility, and set it up again as a fresh backup destination.

Starting a fresh backup means losing your backup history. Only do this if other steps fail and you need backups working again.

When to Call a Professional

If your backup drive consistently disconnects or shows as unreachable, the drive itself may be failing. A technician can test drive health and help you migrate your backups to a new, healthy drive before data is lost.

Frequently Asked Questions

Will I lose my existing backups if I get this error?

No — an error 100008 means the current backup attempt failed, but your previous backups are still intact on the drive. You can still browse and restore files from Time Machine using earlier backup snapshots. Resolve the error and then run a fresh backup to bring everything up to date.

My Time Machine backup drive is on the network (NAS or Time Capsule). What should I check?

Network backups are more prone to this error because any brief network hiccup will cut the connection. First, check that your router and NAS/Time Capsule are on and connected. Then open Finder and see if the network drive appears under Locations in the sidebar. If it does not appear, there is a network connectivity problem to resolve before Time Machine will work.

How do I see what went wrong in the Time Machine backup log?

Hold the Option key on your keyboard and click the Time Machine icon in the menu bar. You will see 'Browse Backup Disks' and also a log option. For more detail, open Console app (in Applications > Utilities), search for 'backupd' and look for error messages around the time the backup failed.