“Preparing backup” and “backup failed” are different problems. Before resetting anything, open System Settings > General > Time Machine, read the message and check when the last backup finished. Your next step depends on what it says.
Still preparing? Give it context first
The first backup can take a long time. Preparation may also take longer after a macOS update, an interrupted backup, or several days without the backup disk. Apple does not give one universal number of minutes after which “preparing” means “stuck.” Keep the Mac powered and the disk connected, then check whether the status changes before you interrupt it.
If it failed, follow the message
- Disk unavailable or disconnected: Check its power and cable. For a local USB disk, connect it directly to the Mac for a test rather than through a hub. For a network disk, check that the Mac and disk are on the same working network.
- Not enough space: Check both the backup disk and the Mac’s startup disk. Apple recommends a backup disk with at least twice the capacity of the data you back up. If the Mac itself is nearly full, Time Machine may lack space for temporary work.
- Some files unavailable: Try a manual backup while the Mac is unlocked. If files are still syncing through a cloud service, let that sync finish while the Mac stays powered and online.
- Read-only disk or disk error: Open Disk Utility and run First Aid on the backup disk. If Disk Utility reports that it cannot repair the disk, stop and decide how to protect your existing backups before considering an erase.
Try the backup again after fixing the specific condition. Avoid deleting snapshots, removing Time Machine settings or erasing the disk just because progress looks slow; those are not useful first checks and can remove a working restore point.
Make sure the next backup really completes
When Time Machine reports success, check its latest backup date. If the same error returns, note the exact wording and whether the disk is local or on a network. A repeating disk error or a disk that disappears from Finder deserves attention before you rely on it as your only backup.
Related: If the Mac’s own storage is almost full, start with our guide to finding large files without a cleaner app.
Sources: Apple Support: If you can’t back up or restore your Mac using Time Machine, Apple Support: If Time Machine remains in the preparing stage, and Time Machine settings. Checked 30 October 2026. This guide reviews Apple’s documented steps; no specific failed backup was reproduced for this article.
