Time Machine Taking Forever to Back Up? Here's What's Slowing It Down

You plugged in the backup drive Friday night, woke up Saturday, and Time Machine is still chugging along at “Preparing backup…” Twelve hours later, the progress bar has barely moved. Something’s wrong, but Time Machine never tells you what.

The honest answer is that Time Machine is rarely “broken” when it’s slow — it’s just doing more work than you think. Indexing 2 TB of small files over USB 2 is genuinely slow. So is backing up an Xcode build folder that regenerates every hour. Below is how to figure out which one you’re hitting and what to do about it.

Why your backup is crawling

Time Machine on macOS Sonoma and Sequoia uses APFS snapshots and copies file-level changes, but a handful of things will tank performance:

• Spotlight indexing the same drive at the same time. Every backup destination gets indexed. If you just connected a new drive or restored from one, mds and mdworker will be eating CPU and disk bandwidth.
• Lots of tiny files. A backup of 50 GB of photos finishes faster than 50 GB of node_modules folders. Small-file overhead is real.
• Network destinations on Wi-Fi. A Time Capsule, NAS, or shared Mac over 5 GHz Wi-Fi caps out around 50–80 MB/s in real-world conditions. The first backup of a 500 GB drive over Wi-Fi can take a full weekend.
• A failing drive. USB drives that are five years old start throwing read errors that show up as “preparing” purgatory.
• Your Mac sleeping. If your Mac sleeps mid-backup and you have “Wake for network access” off, the backup pauses every time the lid closes.

Open Activity Monitor and sort by % CPU. If you see backupd pegged at 100% but the drive’s activity light is barely blinking, you’ve got a small-file problem or a slow link. If mds_stores is also active, Spotlight is the bottleneck.

Check what Time Machine is actually doing

The fastest way to see real progress is the system log. Open Terminal and run:

log show --predicate 'subsystem == "com.apple.TimeMachine"' --info --last 1h

That dumps the last hour of Time Machine activity, including the file count, byte count, and what stage it’s in (Mountpoints, Calculating size, Copying, Finishing). If you see hundreds of “Skipping…” lines for the same path, that’s a signal — usually a permissions issue or a file that’s locked open.

You can also check the next scheduled backup and current state with:

tmutil status

If BackupPhase = Copying and Percent = 0.42 is what you see at hour six, the data really is moving — just slowly.

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Exclude folders that don’t belong in a backup

Time Machine isn’t smart about what’s worth backing up. It’ll happily archive 80 GB of Xcode DerivedData every hour, even though you can rebuild it in seconds.

Open System Settings → General → Time Machine → Options and add these to the exclusion list:

• ~/Library/Caches
• ~/Library/Developer/Xcode/DerivedData
• ~/Library/Developer/CoreSimulator
• Any node_modules, target, build, or dist folders in your dev projects
• Downloads, if you treat it as a scratch folder
• Virtual machine disk images (Parallels, VMware, UTM .vmdk and .vmwarevm bundles) — they’re huge and change every boot
• /Library/Application Support/com.apple.TCC and any Docker.raw files

Also add via Terminal for paths the GUI hides:

sudo tmutil addexclusion -p /path/to/folder

And to confirm what’s currently excluded:

sudo tmutil isexcluded /path/to/folder

The next backup runs a fresh size calculation. If you just trimmed 200 GB of cache directories, expect “Preparing” to take 5–15 minutes while it recomputes.

The first backup is the slow one

If this is your first backup to a new drive, settle in. A full 1 TB initial backup over USB 3.2 Gen 2 SSD takes 3–6 hours. Over Wi-Fi to a NAS, expect 12–24 hours. Subsequent backups are incremental and finish in minutes once the baseline exists.

Don’t unplug the drive mid-backup. Time Machine handles interruption, but if you do it three times in a row, you’ll end up with a .inProgress file that wastes space until the next clean backup completes.

Handle stuck “Preparing backup” states

If Time Machine sits at “Preparing backup…” for more than 30 minutes on an incremental backup, something’s hung. Try these in order:

1. Open Activity Monitor, find backupd and backupd-helper, and force-quit both.
2. Wait 60 seconds, then run tmutil startbackup in Terminal.
3. If it hangs again, eject the drive cleanly and reconnect it.
4. As a last resort, restart the Mac. Don’t unplug the drive without ejecting — that can corrupt the sparsebundle on network destinations.

For network backups specifically, “Preparing” forever usually means the sparsebundle has a lock file. Mount the share manually, look for .com.apple.timemachine.supported, and confirm the destination is healthy. If you see a .inProgress band file older than the current attempt, that’s the culprit.

Reduce what changes between backups

Local APFS snapshots happen hourly and stay on disk for 24 hours, even if your backup drive is unplugged. They eat space on your boot drive and slow the next backup because the engine has to reconcile them.

List current snapshots:

tmutil listlocalsnapshots /

Delete a specific one:

tmutil deletelocalsnapshots 2026-04-28-093000

Or thin them automatically by freeing 50 GB of purgeable space:

tmutil thinlocalsnapshots / 50000000000 4

Time Machine handles this on its own when disk space gets tight, but if your boot drive is over 80% full, the cleanup happens during the backup window and slows everything down.

Reclaim local snapshot spaceSweep finds APFS local snapshots Time Machine left behind. Get Sweep free →

When the drive itself is the problem

Backup drives don’t last forever. If your backup is suddenly 4× slower than it was a month ago, run First Aid on the destination:

1. Open Applications → Utilities → Disk Utility.
2. Select the backup drive (the physical drive, not the volume).
3. Click First Aid.

If First Aid reports problems, the drive is on its way out. Buy a replacement before you need it. Time Machine drives that fail mid-backup are notoriously hard to recover from.

For sparsebundle backups on a NAS, mount the share, then run:

hdiutil verify /Volumes/YourBackup/MachineName.sparsebundle

If it returns errors, your network backup is corrupted. Start a fresh one on a clean directory and keep the old sparsebundle until the new one is verified.

What Time Machine isn’t backing up

Time Machine skips the system volume on macOS Big Sur and later — that’s expected and fine. But it also skips:

• Files marked with com.apple.metadata:com_apple_backup_excludeItem
• iCloud Drive contents that aren’t downloaded locally
• The Trash on every volume
• Anything inside /private/var/vm (swap files)

If you’re missing files in a restore, check whether they were ever local in the first place. iCloud Drive “Optimize Mac Storage” silently evicts files, and Time Machine only backs up what’s physically on the disk.

Clean the boot drive Time Machine ignores

This is where Sweep comes in. Time Machine will faithfully back up 40 GB of cached browser data, old .dmg installers, and Xcode simulator junk every single time those files change — but it won’t ever delete them. That work is on you.

Sweep scans for the categories Time Machine doesn’t manage: caches that Safari and Chrome regenerate constantly, leftovers from apps you uninstalled six months ago, language files for software you only use in English, and the local snapshots that survive past their 24-hour window. Cleaning those before a backup means less data churning every hour.

It’s not a replacement for Time Machine — Time Machine is your time-versioned safety net. Sweep just clears the cruft so the safety net doesn’t have to copy it.

When to start over

If you’ve tried everything and a single incremental backup still takes 8+ hours, it’s time to start fresh:

1. In System Settings → General → Time Machine, remove the current destination.
2. Erase the backup drive in Disk Utility (APFS, GUID Partition Map).
3. Add it back as a destination and let the first backup run overnight.

You’ll lose your version history, which is a real cost. But a backup that doesn’t complete is no backup at all. Keep the old drive disconnected for a month as a fallback, then erase it and use it as a secondary scratch drive.

A healthy Time Machine setup runs hourly in the background and you forget it exists. If you’re noticing it, something’s wrong — and now you know where to look.