To back up an external hard drive on a Mac, copy it to a second drive (or a cloud bucket) with a tool that can tell you afterwards whether every file arrived intact. Time Machine won't do it out of the box. It excludes external drives by default, and it can't back up a drive formatted as exFAT at all.
That surprises a lot of people. Your Mac's internal disk is covered every hour, and the 4TB drive holding three years of footage, which is the drive most likely to fall off a desk, is covered by nothing.
TL;DR: Time Machine excludes external drives by default and can't include exFAT or NTFS drives at all. To back up an external drive, copy it to a second drive with a tool that verifies the copy (Carbon Copy Cloner, ChronoSync, rsync with --checksum, or Tusk), or to the cloud (Backblaze includes external drives but deletes their backup if the drive stays unplugged for 30 days). Then check the copy before you trust it.
Why Doesn't Time Machine Back Up My External Drive?
Time Machine was built to protect your Mac, and it treats anything you plug in as a guest. Open System Settings → General → Time Machine → Options and you'll see every connected external drive already sitting in the "Exclude from backups" list.
For drives formatted as APFS or Mac OS Extended, you can fix that. Select the drive in the list, click the minus button, and click Done. From then on, Time Machine copies the drive every time both it and the Time Machine destination are connected.
For drives formatted as exFAT or NTFS, you can't. Time Machine only backs up APFS and HFS+ volumes, so those drives show up in the exclusion list with the minus button grayed out (Macworld documented this in 2026). exFAT is the format most cameras and most "works on Mac and PC" drives use, which means a large share of creative work lives on drives Time Machine can't touch.
Two more limits matter even when the drive is APFS:
- Your Time Machine disk has to fit both. A 2TB Time Machine drive can't hold your Mac plus a 4TB external, and Time Machine starts deleting its oldest snapshots to make room.
- It only backs up what's connected during a backup. A drive that lives in a bag and gets plugged in twice a month is backed up twice a month, at best.
Four Ways to Back Up an External Drive on Mac
| Method | Works with exFAT | Checks the copy | Cost | Best for |
|---|---|---|---|---|
| Time Machine (after removing the exclusion) | No | No user-facing check | Free | APFS drives that are always connected |
| Clone or sync app (Carbon Copy Cloner, ChronoSync) | Yes | Optional checksum pass | $49.99 one-time (CCC) | A second drive that mirrors the first |
Terminal: rsync | Yes | Yes, with -c | Free | People happy in Terminal |
| Cloud: Backblaze Personal Backup | Yes | Handled by the service | Subscription | An offsite copy of a drive you plug in often |
| Tusk | Yes | Yes, BLAKE3 on every file | $49 for the first 100 licenses, $79 after, one-time | Drives you plug in rarely, several copies, knowing what's where |
1. Copy it to a second external drive
This is the backup most people actually need: a second drive with the same files on it, stored somewhere else (a different room at minimum, a different building ideally).
Finder can do the first copy. Select everything on the source drive, drag it to the second drive, and wait. The problem shows up on the second and third copies: Finder copies everything again, has no idea what changed, and tells you nothing about whether the files that arrived match the ones that left.
If the second drive already has an older, partial copy, our free duplicate file finder can compare both drives in the browser and show what's already there.
A sync or clone app solves the "only copy what changed" part. Carbon Copy Cloner ($49.99) and ChronoSync both handle external-to-external copies on a schedule, and both offer an optional pass that compares file contents.
2. Use rsync from Terminal
rsync ships with macOS and copies only what changed. On macOS 15, the built-in rsync is Apple's openrsync, which accepts the basic flags but not all of them. This copies a drive called Footage to a drive called Footage-Backup, keeping Finder tags (-E):
rsync -aE /Volumes/Footage/ /Volumes/Footage-Backup/
The trailing slash on the source matters: with it, rsync copies the drive's contents; without it, you get a folder called Footage inside the backup. Run the verify command in the section below afterwards.
3. Back it up to the cloud
Backblaze's Personal Backup includes external drives, which not every cloud backup does. The catch is in its own documentation: a drive has to be connected at least once every 30 days. After that, Backblaze treats the drive's files as deleted and removes them from your backup. It emails you at 14, 21, and 28 days. Its paid Extended Version History options remove that limit.
That makes Backblaze a good fit for a drive you use every week and a bad fit for an archive drive in a drawer. For archive drives, an S3-compatible bucket you control (Backblaze B2, Wasabi, Cloudflare R2) keeps files until you delete them. Our B2 vs Wasabi vs R2 comparison has the current prices.
Uploading a full drive takes a while. 2TB over a 50 Mbps upload connection is close to four days of continuous uploading.
4. Use Tusk
Tusk is a Mac app that automatically backs up your project folders to multiple locations and tracks where every file lives — even when your drives aren't connected.
Tusk was built for drives like this. You point it at the drive (it calls this "Track an existing drive"), pick the folders, and it hashes every file with BLAKE3 without moving or re-copying anything. Then you add backup destinations: a second external drive, an S3-compatible bucket, Google Drive, or Dropbox. Tusk copies each file to each destination and checks the hash on arrival.
The part that matters for external drives is what happens after you unplug them. Tusk keeps the record, so you can still see that a file lives on Footage and Footage-Backup and in your B2 bucket, and that one file only exists on the original. There's no 30-day clock. It's Mac only, and it's project and folder based, so a drive full of loose files with no folder structure is a better fit for a disk cataloger.
How to Check That the Backup Actually Worked
Most copy tools decide two files match if they have the same size and the same modification date. That's fast, and it misses exactly the failure you're worried about: a file that arrived damaged.
I tested this on macOS 15.5. I copied three video files to a second folder, then changed a single byte inside one copy without changing its size or date. Here's what each check reported:
| Check | Caught the damaged file? |
|---|---|
rsync -ani (default quick check) | No. Reported nothing. |
rsync -anic (compares contents) | Yes: >fc...... clip2.mov |
diff -rq | Yes: Files src/clip2.mov and d/clip2.mov differ |
shasum -a 256 on both files | Yes, two different hashes |
So after copying a drive, run one of the content checks. This one lists every file that differs between the original and the backup, and changes nothing:
rsync -anic /Volumes/Footage/ /Volumes/Footage-Backup/
No output except folder lines (starting with .d) means every file matches. Expect it to take about as long as the copy did, because it reads every byte on both drives. Tools that verify while copying (Tusk, CCC with its checksum option enabled, offload apps like ShotPut Pro) avoid that second full read.
How Many Copies of an External Drive Do You Need?
Two drives in the same room protect you from a dead drive. They don't protect you from a burglary, a flood, or a spilled coffee that reaches both. The usual answer is three copies: the working drive, a second drive at home, and one copy somewhere else (a cloud bucket, or a drive at a friend's or the office). How to set that up for projects is in our 3-2-1 backup guide, and the best external hard drive guide covers which drives to buy for the copies.
FAQ
Not by default. Time Machine adds every connected external drive to its exclusion list. For APFS or Mac OS Extended drives, open System Settings, General, Time Machine, Options, select the drive, and click the minus button. Drives formatted as exFAT or NTFS can't be removed from the list, because Time Machine only backs up APFS and HFS+ volumes.
Connect both drives and copy the files with a tool that only copies changes and can verify the result: Carbon Copy Cloner, ChronoSync, rsync in Terminal (rsync -aE /Volumes/Source/ /Volumes/Backup/), or Tusk. Finder works for a one-off copy but recopies everything each time and doesn't verify. After the copy, run rsync -anic /Volumes/Source/ /Volumes/Backup/ to list any file whose contents differ.
Yes, just not with Time Machine. Copy tools like Carbon Copy Cloner, ChronoSync, rsync, Backblaze, and Tusk all read exFAT drives. If you want Time Machine to cover the drive, you'd need to copy the data off, reformat the drive as APFS, and copy it back, which also means Windows PCs can no longer write to it.
Backblaze Personal Backup includes connected external drives. A drive has to be reconnected at least once every 30 days, or Backblaze treats its files as deleted and removes them from the backup. It sends warning emails at 14, 21, and 28 days. The paid One Year or Forever Version History options remove the 30-day requirement.
Drive to drive, it depends on the slower drive. A desktop hard drive writes at roughly 150-250 MB/s, so 2TB takes around 2.5 to 4 hours. Two SSDs over USB 3.2 or Thunderbolt can be much faster. To the cloud, upload speed is the limit: 2TB over a 50 Mbps connection is close to four days of continuous uploading.
For a data drive, copy the files. A clone makes sense for a startup disk you might boot from. A data drive only needs its files on another drive, and file-level copies are easier to check, easier to update, and work between drives of different sizes.