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Time Machine NAS Connection Troubleshooting

Issue

Time Machine shows "Connecting to backup disk..." but never completes, even though:

  • ✅ NAS is accessible from Finder
  • ✅ User zjgump is in administrators group (has full permissions)
  • ✅ Network connectivity is fine (ping works, SMB works in Finder)

Root Cause Analysis

Key Finding

zjgump is in the administrators group - This means:

  • Explicit file permissions won't apply to SMB/file protocols
  • Admin users have full access by default
  • The "Permission denied" errors are NOT about file permissions

Actual Issues

  1. Credential Cache Mismatch: macOS has saved credentials for DS224plus (without .local) but Time Machine tries to use DS224plus.local
  2. Stale Mounts: Manual mounts interfere with Time Machine's automatic mounting
  3. Time Machine Process Permissions: Time Machine runs as a different user than your login user

Solutions

Solution 1: Clear Credentials and Let Time Machine Mount Itself (Recommended)

  1. Unmount all existing mounts:

    diskutil unmount "/Volumes/Time Machine Folder" 2>/dev/null
sudo umount -f /Volumes/.timemachine/* 2>/dev/null

  2. Clear saved credentials:

    • Open Keychain Access
    • Search for DS224plus
    • Delete ALL entries related to DS224plus
    • Also search for Time Machine and delete related entries

  3. Remove Time Machine destination:

    • System Settings → General → Time Machine
    • Remove the existing backup disk (click - button)

  4. Re-add Time Machine:

    • Click "Add Backup Disk..."
    • Select "Time Machine Folder" on "DS224plus.local" (the one with .local)
    • When prompted, enter zjgump credentials
    • Let Time Machine mount it itself - don't mount it manually first

Solution 2: Use a Dedicated Time Machine User (Best Practice)

Synology recommends using a dedicated non-admin user for Time Machine:

  1. On Synology DSM:

    • Control Panel → User & Group → Create
    • Create user: tm-backup (or similar)
    • Do NOT add to administrators group
    • Set password

  2. Grant permissions:

    • Control Panel → Shared Folder → Time Machine Folder → Edit → Permissions
    • Add tm-backup user with Read/Write permission
    • Check "Apply to this folder, sub-folders and files"

  3. Configure Time Machine on NAS:

    • Control Panel → File Services → Time Machine
    • Select "Time Machine Folder" as the shared folder
    • Select tm-backup as the user (or leave blank for all users)

  4. On macOS:

    • Remove existing Time Machine destination
    • Add Backup Disk → Select "Time Machine Folder – DS224plus.local"
    • When prompted, use tm-backup credentials (not zjgump)

Solution 3: Fix Credential Cache for Admin User

If you want to keep using zjgump (admin user):

  1. Clear all DS224plus credentials:

    security delete-internet-password -s "DS224plus" 2>/dev/null
security delete-internet-password -s "DS224plus.local" 2>/dev/null

  2. Remove Time Machine destination

  3. Manually connect via Finder first:

    • Finder → ⌘K → smb://DS224plus.local
    • Connect as zjgump
    • This will save the credential properly

  4. Then add Time Machine:

    • System Settings → Time Machine → Add Backup Disk
    • Select "Time Machine Folder – DS224plus.local"
    • It should use the saved credential

Solution 4: Use IP Address Instead of Hostname

If Bonjour (.local) is causing issues:

  1. Find NAS IP:

    ping -c 1 DS224plus.local | grep PING
# Or check your router's DHCP table

  2. Connect via IP:

    • System Settings → Time Machine → Add Backup Disk
    • If it doesn't show up, manually mount first:
      • Finder → ⌘K → smb://192.168.1.172/Time Machine Folder
      • Connect as zjgump
    • Then add the mounted share in Time Machine

Diagnostic Commands

Check Current Mounts

mount | grep -i "timemachine\|ds224plus"

Check Time Machine Logs

log show --predicate 'subsystem == "com.apple.TimeMachine"' --last 10m --info | tail -50

Check Saved Credentials

security find-internet-password -s "DS224plus"
security find-internet-password -s "DS224plus.local"

Test SMB Connection

smbutil status DS224plus.local

Check Time Machine Configuration

tmutil listbackups
defaults read /Library/Preferences/com.apple.TimeMachine

Common Errors and Fixes

"Time Machine could not back up the disk because it is nearly full"

  • Cause: The source disk (Macintosh HD) is nearly full, NOT the destination (NAS)
  • Symptoms: Error says disk is "nearly full" but NAS shows plenty of space (e.g., 1.67TB available)
  • Fix: Free up space on your Mac's internal disk:
    1. Run the diagnostic script: ./free_disk_space.sh
    2. Run the cleanup script: ./fix_time_machine_disk_full.sh
    3. Or manually:
      • Empty Trash: rm -rf ~/.Trash/*
      • Clean caches: rm -rf ~/Library/Caches/*
      • Review Downloads folder (often has large files)
      • Use macOS Storage Management: Apple Menu → About This Mac → Storage → Manage
  • Why: Time Machine needs free space on the source disk to:
    • Create local snapshots
    • Stage files for backup
    • Maintain the backup process
  • Minimum: Aim for at least 5-10GB free space for Time Machine to work properly

"Deleted files but space not freed" / "Stuck Time Machine snapshot"

  • Cause: APFS snapshots (especially Time Machine local snapshots) hold onto deleted file space
  • Symptoms:
    • Deleted 20GB+ of files but disk space didn't increase
    • Error: "Stale NFS file handle" when trying to delete snapshots
    • tmutil listlocalsnapshots / shows snapshots that can't be deleted
  • Fix:
    1. Quick fix - Restart Mac: Often the simplest solution - snapshots are released after restart
    2. Remove Time Machine destination temporarily:
      • System Settings → Time Machine → Remove backup disk
      • Delete snapshot: sudo tmutil deletelocalsnapshots <date>
      • Re-add Time Machine destination
    3. Use the fix script: ./fix_stuck_snapshot.sh (guides you through the process)
    4. Force purge: sudo purge (if available)
  • Why: Time Machine creates local snapshots before backing up. If the backup destination (NAS) is unreachable or has a stale connection, the snapshot gets stuck and holds onto space from deleted files.
  • Prevention: Ensure Time Machine can reliably connect to backup destination

"Permission denied" on mount point

  • Cause: Manual mount with wrong permissions
  • Fix: Unmount and let Time Machine mount it itself

"Connecting to backup disk..." hangs

  • Cause: Credential cache mismatch or stale mount
  • Fix: Clear credentials, unmount all, re-add

"Server may not exist or unavailable"

  • Cause: Using wrong hostname (DS224plus vs DS224plus.local)
  • Fix: Always use .local version or IP address

Prevention

  1. Don't manually mount Time Machine shares - let Time Machine do it
  2. Use dedicated non-admin user for Time Machine (best practice)
  3. Keep credentials in sync - if you change NAS password, update on Mac
  4. Use .local hostname - more reliable than IP or NetBIOS name

Notes

  • Admin users (zjgump) bypass file-level permissions on Synology
  • Time Machine runs as _backup user on macOS, not your login user
  • Bonjour (.local) is preferred over NetBIOS for Time Machine
  • Manual mounts can interfere with Time Machine's automatic mounting
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