How to Back Up Your Mac to an External Drive (Step-by-Step)

You bought an external drive and want to back up your Mac to it. The setup process is genuinely simple, but the steps in the wrong order can leave you with a backup that doesn’t include what you think it does — or one that fails silently three weeks in.

Here’s the complete step-by-step, including the verification you should always do but most guides skip.

Step 1: Get the right drive

If you haven’t bought a drive yet, the rule of thumb: 2× the used space on your Mac, minimum.

• 256 GB Mac with 150 GB used → 1 TB external drive
• 512 GB Mac with 300 GB used → 1 TB drive (minimum); 2 TB recommended
• 1 TB Mac with 600 GB used → 2 TB drive (minimum); 4 TB recommended
• 2 TB Mac → 4 TB drive minimum, 8 TB recommended

The 2× ratio gives you 6-12 months of Time Machine history. With less, Time Machine thins old backups quickly and you lose history you might want.

Drive type:

• SSD — faster, silent, more durable in transport, ~3× more expensive per GB
• HDD — cheaper per GB, slow but acceptable for incremental backups, mechanical so vulnerable to drops

For a MacBook you actually carry, get a USB-C SSD. The Samsung T7 Shield 2 TB is $170 and survives drops. For a desktop Mac, a 4 TB external HDD ($110) is fine — it stays put, speed isn’t critical for incremental backups.

Connection: USB-C (USB 3.2 Gen 1 or Gen 2) is the right pick for any modern Mac. USB-A drives still work via adapters, but new drives should be USB-C.

Step 2: Format the drive correctly

Out-of-box drives often come pre-formatted as exFAT for Windows compatibility. Time Machine won’t use exFAT. You need to format the drive APFS for use with macOS Sonoma or later.

To format:

1. Connect the drive to your Mac
2. Open Applications → Utilities → Disk Utility
3. From the View menu, choose Show All Devices
4. Select the physical drive (not the volume) in the sidebar
5. Click Erase in the toolbar
6. Configure:
   • Name: anything descriptive (“Time Machine Backup”)
   • Format: APFS (or APFS Encrypted if you want password protection)
   • Scheme: GUID Partition Map
7. Click Erase

The format takes 30 seconds for an SSD or a couple of minutes for an HDD. APFS Encrypted requires a password — write it down, because losing it means losing the backup.

Don’t pick HFS+ (formerly Mac OS Extended) unless you need to share the drive with a Mac running macOS High Sierra or older. APFS is the modern format and Time Machine prefers it.

Skip the manual huntSweep clears the system caches, logs, and old installers a backup can’t reach. Download Sweep free →

Step 3: Add the drive as a Time Machine destination

With the drive formatted:

1. Open System Settings → General → Time Machine
2. Click Add Backup Disk
3. Select your drive from the list
4. Click Set Up Disk
5. Choose:
   • Encrypt Backup: yes (highly recommended; requires a password)
   • Disk encryption uses your login password by default

If you already encrypted the drive in Disk Utility (APFS Encrypted), Time Machine inherits that encryption. If you didn’t, this step lets you add it.

Encryption matters: a drive carrying your entire Mac’s data is the same as carrying your Mac itself, security-wise. An unencrypted drive that gets stolen is an instant data leak. Use encryption.

Once added, Time Machine starts the first backup immediately. Don’t unplug the drive.

Step 4: Wait out the first backup

The initial backup is slow because it’s copying everything. Realistic times:

• 200 GB on USB-C SSD: 1.5-2 hours
• 500 GB on USB-C SSD: 3-4 hours
• 1 TB on USB-C SSD: 6-8 hours
• 200 GB on USB 3.0 HDD: 2-3 hours
• 1 TB on USB 3.0 HDD: 8-12 hours
• Anything over Wi-Fi: don’t bother, plug the drive in

Plug your Mac into power. Don’t put it to sleep. If you’re on a MacBook, set “Prevent automatic sleeping when display is off” in System Settings → Lock Screen to “Always.”

Run the first backup overnight. It’ll be done by morning.

After the first backup, subsequent backups are incremental. They run hourly automatically and take 1-10 minutes for typical changes.

Step 5: Configure exclusions

Once the first backup completes, set up exclusions so Time Machine doesn’t waste space and time on stuff that doesn’t belong in a backup.

System Settings → General → Time Machine → Options

Add to the exclusion list:

• ~/Library/Caches (regenerated automatically)
• ~/Library/Developer/Xcode/DerivedData (if you use Xcode)
• ~/Library/Developer/CoreSimulator (Xcode iOS simulators)
• ~/Library/Containers/com.docker.docker (Docker data)
• ~/Parallels or ~/Virtual Machines.localized (VM disk images)
• Any node_modules, target, build folders in dev projects
• Browser caches: ~/Library/Caches/Google/Chrome, ~/Library/Caches/com.apple.Safari
• Spotify, Slack, Discord caches in ~/Library/Application Support/[app]

Add via the GUI + button or via Terminal:

sudo tmutil addexclusion -p ~/Library/Caches

Be careful what you exclude. Don’t exclude:

• ~/Documents — your work
• ~/Desktop — files in progress
• ~/Pictures/Photos Library.photoslibrary — irreplaceable
• ~/Library/Mail — local message store
• ~/Library/Keychains — passwords and certificates

Step 6: Verify the backup actually works

The most important step everyone skips. A backup that hasn’t been tested isn’t a backup.

To verify:

1. Confirm the backup completed: open Time Machine menu bar → check for a green checkmark or recent timestamp
2. Run tmutil status in Terminal — Result = 0 means clean
3. Run tmutil listbackups — confirm you see today’s backup
4. Try a restore: open a folder, click Time Machine in the menu bar, scroll to a recent point, restore one file
5. Open the restored file and confirm it’s intact

If any of those steps fail, fix the underlying issue before continuing. A backup that “completed” but can’t actually restore is worse than no backup at all because you’ll trust it.

Step 7: Set up the routine

Time Machine runs automatically on its hourly schedule, but you can change behavior:

• Connection-based: by default, Time Machine backs up whenever the destination is connected. For an always-connected drive (desktop Mac), this is set-and-forget.
• Specific schedule: macOS Sonoma added options for “Every Hour,” “Every Day,” “Every Week,” and “Manually” via System Settings → General → Time Machine → Options
• Wake for Wi-Fi access: if your destination is on a network, enabling this in Battery/Energy settings lets Time Machine back up while the Mac is asleep

For most users, hourly automatic is fine. For battery-conscious MacBook users, “Every Day” is more reasonable.

Step 8: Plan for the drive failing

External drives don’t last forever. Build redundancy from day one.

Options:

• Two drives, alternating: one connected, one stored offsite. Swap monthly.
• Two drives, both connected: Time Machine can use multiple destinations. Add both in System Settings → General → Time Machine.
• One drive plus cloud backup: external for fast restore, cloud for disaster recovery
• NAS as one of the destinations: more complex but allows backup over the network

The minimum: have at least one backup that doesn’t live next to your Mac. If a fire or theft takes the Mac, you don’t want it to take the only backup too.

For “two drives, both connected,” Time Machine alternates between them. Each backup goes to one destination. Over time, both have full history but slightly different snapshot timing. If one dies, the other has you covered.

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

Manual backups via rsync

If you want a non-Time Machine backup as a secondary, rsync is the classic Unix tool. macOS ships with an old version (2.6.9), so install a current version with Homebrew:

brew install rsync

Basic backup of your home folder:

rsync -avh --delete \
  --exclude='Library/Caches' \
  --exclude='Library/Containers/com.docker.docker' \
  --exclude='node_modules' \
  ~/ /Volumes/BackupDrive/home/

Flags:

• -a archive mode
• -v verbose
• -h human-readable
• --delete removes files on destination that don’t exist on source

Schedule with launchd or cron, or just run manually before big changes.

rsync gives you a plain-file copy. No versioning, no fancy interface, but readable on any system. Good as a secondary to Time Machine.

For versioning, look at restic (Homebrew installable) — encrypted, deduplicated, versioned backups to local or cloud destinations.

What your backup misses

Even a perfect Time Machine setup misses things:

• External drives — Time Machine doesn’t back up other external drives by default. If you have a working drive with critical data, back it up separately.
• iCloud-only files — files set to “Optimize Mac Storage” might be in iCloud only, not local, and not in your Time Machine backup
• Files created and deleted between backup intervals — Time Machine snapshots hourly; a file that lives less than an hour might never be in a backup
• macOS system itself — backed up but not bootable on Apple Silicon (rebuild via Recovery if needed)

For external drives, run a separate backup. Either Time Machine to a different destination, or rsync them to a backup drive periodically.

Why a clean Mac backs up better

External drive backups copy whatever’s on your Mac. If your Mac has 60 GB of cache cruft, that’s all in every backup, eating space on the destination.

Sweep cleans the categories Time Machine doesn’t manage:

• System and user caches that regenerate constantly
• Old .dmg and .pkg installers in Downloads
• App leftovers from drag-to-Trash uninstalls
• Localizations for languages you don’t speak
• Old iOS device backups
• Local APFS snapshots eating boot-drive space

It doesn’t replace Time Machine. Time Machine is the version-history insurance. Sweep just keeps the boot drive lean so backups copy real files instead of cruft.

A 500 GB Mac that’s actually 250 GB of real data plus 250 GB of cache will need a 1 TB backup drive. The same Mac with 50 GB of cache (after a Sweep clean) needs a 600 GB backup drive. Smaller backups, faster backups, longer history on the same destination.

A complete backup checklist

For someone setting this up from scratch:

• Buy 2× external drive (HDD or SSD based on use case)
• Format APFS, GUID Partition Map, encrypt
• Add as Time Machine destination
• Run first backup overnight
• Configure exclusion list (caches, dev tools, VMs)
• Wait for incremental backups to start running automatically
• Test a restore: pull a random file, verify it works
• Set quarterly calendar reminder for restore test
• Add second backup destination (cloud or second drive) for redundancy
• Document the password if drive is encrypted (somewhere safe)

That’s a one-Saturday project. The rest is automated.

What ongoing maintenance looks like

Once everything’s set up:

• Daily: nothing. Time Machine handles itself.
• Weekly: glance at the menu bar icon — green checkmark means backups are working
• Monthly: check tmutil listbackups confirms regular backups are happening
• Quarterly: test a restore (see step 6)
• Yearly: review the exclusion list for new apps or cache folders

External drive lifespan is 4-7 years. Replace HDDs at 5 years; replace SSDs when SMART warns or they’re 6+ years old. Keep the old drive offline as a fallback for 60 days after switching to a new one.

A working backup is invisible. You set it up once, test it occasionally, and forget it exists — until the day you need it. That’s the goal. The 90 minutes you spend setting this up properly are the cheapest insurance you’ll ever buy.