Guide

How to Automatically Back Up an External Drive on Mac

A backup that only happens when you remember to plug in a drive and click a button isn't really automatic — it's manual with extra steps. Here's how to make it actually hands-off, and what your real options are.

Updated August 9, 2026.

Key takeaway

"Automatic" has to mean your Mac notices the drive and acts on its own, not just that a capable tool exists. Two real automatic options: Time Machine (built in, versioned, but locked inside its own backup format) and a mirror tool like ThirdCopy (a plain, browsable copy of exactly the folders you choose, watching for the drive to reconnect). FreeFileSync can do this too and is free, but on Mac it has no built-in scheduler, so "automatic" means writing your own cron job or launchd file.

Why manual backups to an external drive quietly stop happening

The failure mode here isn't dramatic — nobody forgets on purpose. It's just that a backup depending on two separate human actions, plugging in the drive and then remembering to actually run something, has two separate points where it can silently not happen. Skip a week because you're traveling, or busy, or the drive is buried in a bag, and there's no backup for that week. Do that a few times and it's not unusual to find a "backup" drive that's actually two or three months stale. The fix isn't a better reminder. It's removing the human step, so a drive connecting is what triggers the backup, not a step that comes after that.

Two ways to make it actually automatic

Time Machine

If your external drive is formatted correctly and you've selected it as a Time Machine disk in System Settings, macOS backs up to it automatically and continuously whenever it's connected — no scheduling required. That's a genuinely good default for whole-Mac disaster recovery: if your Mac's internal drive dies, Time Machine gets you back to a working state, with real version history letting you dig up a file as it existed hours or weeks ago. The tradeoff is that everything lives inside Time Machine's own backup format, on its own volume, not as a folder of plain files you can browse in Finder. Wanting one specific file back means going through Time Machine's restore interface, not just opening the drive.

A mirror tool like ThirdCopy

A mirror tool takes a narrower approach: instead of backing up your whole Mac into a proprietary format, it watches the specific folders you chose and keeps a plain, browsable copy of them on the drive. ThirdCopy specifically watches for the destination drive to reconnect and picks up automatically — either continuously while it's plugged in, or on a schedule, with a “back up when the destination connects” option that covers the common case of a drive that isn't always plugged in. Because the result is just files in folders, not a container format, you can open the backup drive in Finder and see exactly what's there.

The free DIY path: FreeFileSync

FreeFileSync deserves a fair mention here: it's free, open-source, and even has a mode literally called "Mirror." Where it falls short of "automatic" on a Mac specifically is scheduling — FreeFileSync on Mac has no built-in scheduler. Making it run automatically means writing and installing your own cron job or launchd property list yourself, which is a real, non-trivial barrier if you're not comfortable in Terminal. That's not a knock on FreeFileSync's engine, which does real work for free — it's just an honest note that "free and capable" and "automatic and hands-off for a non-technical user" aren't the same claim. Full detail in the FreeFileSync comparison.

Setting it up with ThirdCopy

Prepare the drive. If it's only ever going to connect to a Mac, format it APFS. If it might ever plug into a Windows PC or another non-Mac device too, use exFAT instead so it stays readable there. Either works fine as a ThirdCopy destination.

Run the 3-step setup. Pick the folders you want backed up, pick the external drive as the destination, and pick continuous mode or a schedule — hourly, every 6 hours, daily, weekly, or manual.

Unplug it without worrying. Scheduled and continuous runs both simply wait: ThirdCopy detects that the destination isn't currently connected, skips that run rather than failing loudly, and picks back up automatically the next time the drive reconnects.

One more thing worth knowing

An external drive that lives permanently connected to your Mac is convenient, but it isn't really a separate backup in the fullest sense. Theft, a power surge, a spilled drink, or a fire takes out your Mac and the "backup" sitting right next to it together. For real redundancy, the drive should sometimes be disconnected or rotated with a second one, or paired with a NAS and an offsite or cloud copy — see the 3-2-1 backup rule guide for the full shape of that.

Questions

Does Time Machine back up automatically to an external drive?

Yes. Once an external drive is formatted correctly and selected as a Time Machine backup disk in System Settings, macOS backs it up automatically and continuously whenever the drive is connected, with no schedule to configure. The tradeoff is that the backup lives inside Time Machine's own backup format rather than as plain, browsable files.

What format should my external backup drive use?

It depends on what else the drive needs to talk to. If it's a Mac-only drive, APFS is the modern default — fast, journaled, and what Time Machine will typically want to format a blank disk as. If the same drive ever needs to plug into a Windows PC or another non-Mac device, use exFAT instead, since APFS isn't natively readable outside the Apple ecosystem and exFAT is close to universal. A plain mirror tool works fine with either format — the choice comes down to where else the drive needs to work, not the backup software.

Can I back up to an external drive that's not always connected?

Yes — that's the common case, not an edge case. Time Machine simply resumes backing up the next time the drive is connected, picking up from where it left off. A mirror tool like ThirdCopy handles it the same way: a scheduled or continuous backup whose destination isn't currently connected is skipped, not treated as a failure, and it runs automatically as soon as the drive reconnects, including a “back up when the destination connects” option built for drives that are only occasionally plugged in.

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