macOS Time Machine Error 256 — Backup Could Not Be Completed Fix

What Does This Error Mean?

Time Machine Error 256 means the backup could not be completed due to an input/output error — which usually means macOS could not read a file on your Mac or write to the backup drive. This error is often caused by a file your Mac cannot access, a corrupted backup, or a problem with the backup drive itself.

Affected Models

MacBook Air
MacBook Pro
iMac
Mac Mini
Mac Studio

Common Causes

A specific file on your Mac cannot be read — it may be corrupted or locked by an app
The backup drive has bad sectors or file system errors causing write failures
The Time Machine backup sparsebundle is corrupted
The backup drive ran out of space mid-backup
A network drive used for backup had a connection interruption during the backup

How to Fix It

Run First Aid on the backup drive. Open Disk Utility (Applications > Utilities). Select your Time Machine backup drive and click First Aid. Let it complete.

An I/O error when writing to the drive often indicates file system errors. First Aid can repair many of these.
Check the Console log for the specific file causing the error. Open Console app (Applications > Utilities > Console). Filter by 'TimeMachine' or 'backupd.' The log will name the exact file that failed.

Knowing which file failed helps enormously. Sometimes a single problematic file can be excluded from Time Machine backups to allow the rest to succeed.
Exclude the problem file from Time Machine. Go to System Settings > General > Time Machine > Options. Click the + button and add the file or folder causing the error to the exclusion list.

If a file is locked by an app, Time Machine cannot back it up. Excluding it from backups lets everything else proceed normally.
Delete the incomplete backup. Open your backup drive in Finder. Navigate to Backups.backupdb and find any folder ending in .inProgress. Delete it.

Incomplete backup files can block subsequent backups. Deleting them lets Time Machine start a fresh backup.
Start a new backup if needed. Go to System Settings > General > Time Machine. Remove the backup destination and add it back. Then manually start a backup using Back Up Now.

Starting fresh loses your old backup history but gives you a clean slate free of any corruption.

When to Call a Professional

If Error 256 appears consistently and Disk Utility First Aid cannot repair the backup drive, the drive may be physically failing. Back up your data to a different drive as soon as possible. An Apple Authorized Service Provider can advise on drive replacement if needed.

Frequently Asked Questions

What is an input/output error?

An input/output (I/O) error means the drive could not successfully read or write data. It is like trying to write on paper and the pen keeps skipping — data is getting lost in the process. This can be caused by a bad sector on the drive, a failing drive, or a faulty cable.

Can I restore from a backup that ended in Error 256?

Possibly yes — an incomplete backup may still contain most of your files up to the point it failed. However, files that were being written at the moment of the error might be corrupted. Try entering Time Machine and browsing to see how recent the last successful backup was.

How can I tell if my backup drive is failing?

Signs of a failing backup drive include: frequent I/O errors, First Aid finding errors it cannot repair, the drive taking longer and longer to appear after plugging in, and unusual noises (clicking or grinding). A drive showing multiple signs should be replaced promptly.