What's happening
Time Machine throws Time Machine couldn't complete the backup. The backup disk could not be found. when it can't reach the backup destination at scheduled backup time. The usual suspects: a USB drive was unplugged, a NAS went offline, your Mac quietly switched Wi-Fi networks, or macOS lost the disk association after an update.
The fix depends on your setup — USB drive, network volume, or NAS. Work through the steps below in order. Most cases resolve at step 1 or 2.
Step 1 — Verify the backup disk is visible to macOS
Open Disk Utility (Applications → Utilities → Disk Utility) and check whether your backup drive appears in the sidebar.
- USB drive not listed: unplug, wait 10 seconds, replug. Try a different port — some USB hubs lose power state after sleep.
- NAS or Time Capsule missing: confirm the device is powered on and connected to the same network as your Mac.
From Terminal, list mounted volumes:
ls /Volumes/
Your backup disk name should appear there. If it doesn't, macOS can't see it yet. Fix the physical or network connection first — the remaining steps won't help if the drive isn't visible.
Step 2 — Reconnect the backup disk in Time Machine settings
A drive can be perfectly visible to macOS and still fail Time Machine. After macOS updates, the backup association sometimes silently breaks.
- Open System Settings → General → Time Machine (macOS Ventura+) or System Preferences → Time Machine (older).
- Click the backup disk entry. See a warning icon? Remove it with the − button.
- Click Add Backup Disk… and reselect your drive.
Force an immediate backup to confirm it works:
tmutil startbackup --auto
Watch the status in real time:
tmutil status
Step 3 — Fix network/NAS volume mounting
Synology, QNAP, and other SMB shares need to be mounted before Time Machine can back up to them. macOS won't do this automatically unless you set it up. If you're on Wi-Fi, also confirm your Mac didn't quietly switch to a guest network or hotspot.
Manually mount the NAS share
In Finder: Go → Connect to Server (⌘K), then enter the address of your NAS:
smb://192.168.1.x/TimeMachineShare
# or for AFP (older devices):
afp://192.168.1.x/TimeMachineShare
Once mounted, trigger a backup:
tmutil startbackup --auto
Make the share mount at login
Go to System Settings → General → Login Items and add the mounted network volume. Without this, Time Machine will fail every reboot — the disk simply won't be there when the first scheduled backup fires at 2 AM.
Step 4 — Reset the Time Machine backup database
Disk reachable, association looks correct, but backups still fail? The local snapshot database may be corrupted. This happens occasionally after hard shutdowns or interrupted backups.
# Stop Time Machine
sudo tmutil disable
# Clear the verification cache and status plist
sudo rm -rf /private/var/db/com.apple.backupd.backupVerification
rm -f ~/Library/Preferences/com.apple.TimeMachineStatus.plist
# Re-enable
sudo tmutil enable
Reassociate the disk (Step 2) and trigger a backup to verify.
Step 5 — Check and repair the backup disk itself
Here's a non-obvious behavior: a disk with filesystem errors can make Time Machine report "disk not found" rather than a proper disk error. Run First Aid before assuming the drive is healthy.
# Find your disk identifier
diskutil list
# Run First Aid (replace disk2s1 with your actual identifier)
diskutil repairVolume /dev/disk2s1
You can also do this from Disk Utility GUI: select the backup volume → First Aid → Run.
If First Aid reports errors it can't repair, check SMART status:
diskutil info /dev/disk2 | grep SMART
SMART Status: Verified means the drive hardware is fine. Any other result — start shopping for a replacement before you lose everything.
Step 6 — Rebuild from scratch (last resort)
Nothing above fixed it? Wipe the association and start fresh. You'll lose backup history, but a clean backup beats no backup.
# List current backup destinations
tmutil destinationinfo
# Remove the broken destination (use the ID from destinationinfo output)
tmutil removedestination <destination-id>
# Erase and reformat the backup disk via Disk Utility
# Format: Mac OS Extended (Journaled) for local USB drives
# macOS Big Sur and later also support APFS for Time Machine
# Re-add it
tmutil setdestination /Volumes/YourBackupDiskName
Verify the fix
# Start a backup and watch the status
tmutil startbackup
tmutil status
# Confirm the latest backup completed
tmutil latestbackup
# List all backup snapshots
tmutil listbackups
After completion, tmutil status shows Running = 0. Run tmutil latestbackup — it should return today's date.
Lessons learned
- Mount NAS shares at login. Time Machine runs on a schedule, not on demand — if the share isn't mounted when backup fires at 2 AM, you get this error. Login Items solve it permanently.
- Always check Time Machine after a major macOS update. Upgrades silently break backup associations more often than Apple would like to admit. Two minutes in System Settings can save hours of recovery pain.
- Format USB backup drives as Mac OS Extended (Journaled), not APFS — unless you're on macOS Big Sur or later, where APFS is fully supported for Time Machine.
- Don't wait for SMART to fail. A drive reporting intermittent errors is already in decline. Replace it before a mid-backup failure leaves you with no usable restore point.