Time Machine Troubleshooting

I often see folks posting issues about using Apple's Time Machine with their Macs so I thought it might be a good time to create this tip on how you can perform some initial troubleshooting yourself before posting. In most cases you will be able to resolve issues quickly and learn a bit along the way. As an added bonus, you can print this out for future reference.

Time Machine Troubleshooting Decision Tree

START: “Time Machine is failing / stuck / not completing”

Step A — Identify the destination type

Is the destination…

• External drive directly connected to the Mac? → go to Branch USB
• NAS (Synology/QNAP) over network? → go to Branch NAS

Branch USB (External Drive / Direct Attached)

USB-1: Does Finder see the drive reliably?

Check: Finder → Locations → drive appears

• NO (missing / randomly disconnects)
  • ✅ Likely cause: cable/hub/enclosure/power
  • ✅ Actions:
    • connect directly (no dock/hub)
    • change cable
    • try different port
    • if bus-powered HDD → try powered hub or different enclosure
• STOP (fix stability first)
• YES → continue

USB-2: Can the Mac write to the drive normally?

Test: create folder TM_TEST, copy small file

• NO (permission/read-only/errors)
  • ✅ Likely cause: filesystem damage or mount issues
  • ✅ Actions:
    • Disk Utility → View → Show All Devices
    • First Aid on backup volume
    • check free space
• STOP until resolved
• YES → continue

USB-3: Can it write a large file (5GB+)?

Test: copy a 5GB+ file to the drive

• NO (copy fails / stalls / disconnects)
  • ✅ This is NOT “a Time Machine problem yet”
  • ✅ Likely cause: physical disk, cable, bridge chipset, or filesystem issues
  • ✅ Actions:
    • try different cable/port
    • Disk Utility First Aid
    • consider replacing disk/enclosure
• STOP
• YES → continue

USB-4: Time Machine still fails?

Now it’s likely Time Machine metadata, snapshots, or OS-level behavior

Next best checks:

• Is it stuck at “Preparing backup…”?
  • YES → Branch PREP
• Does it fail during copy phase (after progress starts)?
  • YES → Branch COPY
• Does it complete but restores are broken?
  • YES → Branch RESTORE

Branch NAS (Synology / QNAP over SMB)

NAS-1: Can Finder reliably connect/mount the NAS share?

Check: Finder → Locations → NAS → share stays connected

• NO
  • ✅ Likely cause: network instability, NAS sleeping, Wi-Fi issues, bad name resolution
  • ✅ Actions:
    • test Ethernet if possible
    • reboot NAS + router (if appropriate)
    • confirm Mac can access share consistently
• STOP
• YES → continue

NAS-2: Can it write a small file?

Test: create TM_TEST folder on the share

• NO
  • ✅ Likely cause: share permissions / credential issue
  • ✅ Actions:
  • remove/re-add credentials (Keychain Access)
  • reconnect to NAS
• STOP
• YES → continue

NAS-3: Can it write a large file (5GB+)?

Test: copy a 5GB+ file to the share

• NO
  • ✅ Likely cause: SMB/network dropouts or NAS storage problem
  • ✅ Actions:
    • if Wi-Fi → test Ethernet
    • check NAS free space + health
  • reduce network complexity (temporary)
• STOP
• YES → continue

NAS-4: Time Machine still fails?

Now it’s likely sparsebundle/Time Machine structure, verification issues, or backups “poisoned” over time.

Next best checks:

• Is it stuck at “Preparing backup…”?
  • YES → Branch PREP
• Does it fail mid-backup repeatedly?
  • YES → Branch COPY (NAS flavor)
• Does it succeed but can’t restore?
  • YES → Branch RESTORE

Branch PREP (Stuck at “Preparing backup…”)

This is almost always scan/churn/indexing/snapshots.

PREP-1: Is the Mac doing heavy background work?

Check: Activity Monitor

• mds / mdworker high → Spotlight indexing: wait it out (or reduce churn)
• photoanalysisd high → Photos analysis (can slow TM)
• backupd high CPU with no progress → scanning lots of small changes

✅ Actions:

• remove huge churn folders from backup (VMs/caches/dev folders)
• confirm enough free space (source volume)
• restart Mac and try again

If still stuck after repeated attempts:

• run Disk Utility First Aid on Macintosh HD
• consider a new backup set (if the destination is old and messy)

Branch COPY (Fails during copy / after progress begins)

This is usually destination instability or corruption.

COPY-1: Does a 5GB manual copy succeed?

• NO → it’s connection/disk/network first (not TM)
• YES → likely TM metadata/verification failure

✅ Actions:

• Disk Utility First Aid on destination
• if NAS: check NAS storage health + free space
• if external: change cable/enclosure
• if repeated failures and destination is old: start a new backup set

Branch RESTORE (Backup completes but restore/browsing fails)

This is where you verify real usability.

Best test:

• Restore a single small file from yesterday
• Or test in Migration Assistant

If restore fails:

• treat backup as unreliable
• run First Aid
• often best outcome is starting a fresh backup set once disk health is confirmed

Quick Reference: Symptoms -> Likely Causes

TM Process Phases

Phase 0: Pre-Flight (Before TM Even Starts)

Symptoms

• Backup never starts (“Preparing backup…” forever or nothing happens)
• TM shows “Waiting to complete first backup”
• Backup destination shows as available, but no progress occurs
• TM can’t find the disk (even though it’s mounted)
• “Time Machine couldn’t complete the backup” immediately, no detail

Likely Causes

• Backup disk not mounted reliably / unstable path
• TM destination “registered” but not actually writable
• Wrong disk format (SMB shares especially)
• Sleep/power/network is interrupting the pre-checks
• macOS background tasks are blocking or starving TM
• Corrupted TM destination metadata (local or network)

Phase 1: Destination Discovery & Access

Symptoms

• “Backup Disk Not Available”
• “Network backup disk disconnected”
• Backup starts only when you manually open the disk in Finder
• Works sometimes, fails other times (intermittent)
• Slow initial connection to NAS/SMB/AFP legacy targets

Likely Causes

Local disks

• USB-C/USB bridge chipset instability
• Insufficient power (bus-powered HDDs especially)
• Bad cable, flaky hub/dock
• Drive enclosure overheating or sleeping aggressively

Network destinations (SMB)

• Wi-Fi roaming / unstable latency
• DNS or name resolution issues (especially with NAS hostnames)
• SMB signing/encryption issues in some environments
• NAS going to sleep, spinning down, or power-saving NIC state
• Credential/keychain problems

Phase 2: "Preparing Backup"

Symptoms

• Stuck on “Preparing backup…” for hours
• CPU high, disk thrashing, no progress bar
• “Finding changed files” never finishes
• Backup sizes seem incorrect (too large/small)
• Backup constantly re-backs up “everything”

Likely Causes

• Spotlight indexing load (source or destination)
• Massive file count changes (dev folders, VMs, node_modules, caches)
• APFS snapshot issues on the source volume
• Corrupt local snapshots
• File system errors on source volume
• Millions of small files (Photos libraries, Mail, Xcode, etc.)

Phase 3: Snapshot Creation (APFS Local Snapshot)

Symptoms

• Backup fails instantly after starting
• Errors mentioning snapshots
• TM says backup completed but data seems inconsistent (rare)
• “Could not create a snapshot” type behavior in logs

Likely Causes

• APFS volume out of free space (snapshots need breathing room)
• APFS metadata issues
• Source disk problems
• Third-party tools aggressively manipulating snapshots

Phase 4: Backup Copy (Data Transfer & Incremental Writes)

Symptoms

• Backup gets partway and fails every time
• Slow backup speed (USB is slow, or SMB is slow)
• “Time Machine couldn’t complete the backup” with vague error
• Disk disconnects mid-backup
• Destination becomes read-only unexpectedly

Likely Causes

• Destination disk corruption or failing hardware
• Network instability (SMB dropouts)
• NAS or router doing power-save/EE settings
• USB controller resets
• Drive being ejected due to “improper disconnect”
• Antivirus/security software interference (less common on macOS, but exists)

Phase 5: Verification / Thinning / Cleanup

Symptoms

• Backup “finishes” but disk fills up quickly
• “Deleting old backups…” hangs
• Backups won’t complete due to insufficient space
• “Verification failed” / “Inconsistent” type behavior

Likely Causes

• Destination too small for dataset + retention
• TM thinning stuck on huge file churn
• Directory/index corruption on the destination
• NAS sparsebundle problems (network backups)
• APFS destination volume issues

Phase 6: Post-Backup

Symptoms

• Entering TM is slow / empty
• Can’t browse older backups
• Restore fails with permissions issues
• Migration Assistant can’t see the backup
• Backup exists but files are missing

Likely Causes

• Incomplete backup set due to past failures
• Customer assumes something was backed up but it was excluded
• Permissions/TCC restrictions preventing access to some areas
• Backup disk index corruption
• Network sparsebundle mount instability