Time Machine Couldn't Complete the Backup or Backup Disk Not Available? How to Fix It

Published ✓ Verified ⏱️ 3 min read July 19, 2025 · by Ryan Bennett

Time Machine reports that it couldn’t complete the backup, says the backup disk isn’t available, or can’t find your disk at all.

Why: Usually the disk has come loose, gone to sleep, dropped off the network, or run low on space — or another app (security software, a VPN, virus scanning) is interfering. Apple’s guidance is to rule these out one at a time.

Fix 1: Check the disk and its connection

If the disk connects by cable, make sure it’s plugged in firmly, connected directly to your Mac — not through a USB hub — powered on, and shows up in Finder.
If you back up over your network, confirm the network connection is good and your Mac is on the same network as the backup disk.
Open Apple menu > System Settings > General > Time Machine and confirm the disk is still listed. If a network disk is missing, it may be off, asleep or temporarily unreachable — wake it and try again.

Fix 2: Make sure there’s enough space

Apple recommends a backup disk at least twice the storage capacity of your Mac. If the disk is nearly full, see the Time Machine disk is full.
Check your startup disk too — if it’s nearly full, Time Machine can’t save the temporary data it needs to finish a backup. Free up space if needed (see free space when the startup disk is full).

Fix 3: Repair the backup disk with First Aid

A read-only or corrupted disk often shows up as a failed backup.

Open Disk Utility (Spotlight, or Applications > Utilities).
Select your backup disk in the sidebar.
Click First Aid, then Run, and let it finish.

If First Aid can’t repair the disk, you may need to erase it and start a fresh backup, or use a different disk.

Fix 4: Rule out interference, then retry

Exclude the backup disk from virus scanning in any third-party security software — scanners can lock files Time Machine needs.
Temporarily turn off a VPN or third-party firewall and try again.
If the message said the backup was delayed because some files were unavailable, complete a manual backup while your Mac is unlocked (choose Back Up Now from the Time Machine menu), or just wait — Time Machine retries automatically on the next scheduled backup.

FAQ

It’s stuck on “Preparing backup” for ages. The first backup, or a backup after big changes, takes a long time to index. Leave the Mac on and plugged in. If it never moves, work through Fixes 1–3 above.

Do I lose old backups if I repair or erase the disk? Erasing the disk deletes its backups. Only erase as a last resort, and ideally keep a second backup disk.

Sources: Apple Support — If you can’t back up or restore your Mac using Time Machine · Apple Support — If a Time Machine backup fails on Mac