Guide
Mirror vs. sync vs. clone: what's the difference?
Mirror, sync, and clone get used as if they're interchangeable. They're not — each one makes a very different promise about what you can get back, and when. Here's what each actually does, in plain language, for a Mac and NAS setup.
Updated August 9, 2026.
A mirror makes the destination match the source exactly, with no history — delete something at the source and it's gone from the mirror too. A two-way sync lets either side change and pushes that change to the other, at the cost of occasional conflicts. A clone is a bootable copy of your whole startup disk — a category Apple Silicon has made increasingly impractical. A versioned backup (like Time Machine) keeps history, so you can go back to last Tuesday, not just today. Most people end up wanting a mirror and a versioned backup together — they solve different problems, not the same one twice.
Mirror: one-way, no history
A mirror makes the destination match the source, and nothing more. New and changed files get copied over; files that were deleted at the source get removed from the destination on the next run too. There's no history — a mirror always reflects the current state of the source, not any past state. That's exactly what ThirdCopy does, and it's what FreeFileSync calls its "Mirror" mode.
The appeal is that it's simple and predictable: what's on the destination is always exactly what's on the source, no more and no less. The honest tradeoff is the one people don't think about until it happens — if you delete something at the source by mistake and don't notice before the next run, it's gone from the mirror too. Some mirror tools soften this with an optional safety net: ThirdCopy can hold deleted files for a period instead of removing them immediately, but it's opt-in, and it's a cushion, not a substitute for real version history.
Two-way sync: changes flow in both directions
Sync tools watch two locations and propagate changes either way — edit a file on your laptop and it shows up on your desktop, and vice versa. That's genuinely useful when you actively work from both locations and want them to stay identical, rather than one being a passive copy of the other.
The real risk is conflicts: if the same file changes on both sides between syncs, the tool has to decide what "winning" means, and it's not always obvious which version you actually end up with. ChronoSync, GoodSync, and FreeFileSync's "Two-way" mode all handle this job. A plain mirror tool deliberately doesn't offer two-way sync at all — one direction only means there's no conflict to resolve, by design, not by omission.
Clone: a bootable copy of your whole disk
"Clone," in Mac backup terms, usually means a bootable clone: a full copy of your entire startup disk that you could, in theory, boot your Mac from if the internal drive failed. This was the headline feature of tools like Carbon Copy Cloner and SuperDuper! for years.
Worth knowing, factually: Apple Silicon's security architecture requires the internal drive's own recovery environment to cryptographically bless anything booted externally, which has made true bootable external clones unreliable to impractical across the board on recent Macs. That's a platform-wide constraint, not a specific tool falling short. For most people, a plain file-level backup plus macOS's own reinstall and Migration Assistant path now gets you back up and running more reliably than chasing bootability does.
Side by side
| Mirror | Two-way sync | Clone | |
|---|---|---|---|
| Direction | One-way, source → destination | Two-way, either side → the other | One-way, source → destination (whole disk) |
| History kept | None — reflects the source's current state only | Usually none | None — a single point-in-time snapshot |
| Deleted files | Removed from the destination too | Removed on both sides, or flagged as a conflict | N/A — a full-disk copy, not incremental |
| Conflicts possible | No — only one direction exists | Yes — same file changed on both sides needs a resolution rule | No |
| Typical job | An always-current duplicate of a folder, off the Mac | Keeping two active locations (laptop + desktop) consistent | An emergency boot disk if the internal drive fails |
| Example tools | ThirdCopy, FreeFileSync (Mirror mode) | ChronoSync, GoodSync, FreeFileSync (Two-way mode) | Carbon Copy Cloner, SuperDuper! |
The fourth concept: versioned backup
Worth naming even though it's not in the title: a versioned, or incremental, backup keeps a history of past states, so you can restore a file as it looked last Tuesday, not just as it looks right now. Time Machine and Arq both work this way.
The tradeoff against a mirror is straightforward — more recoverability, at the cost of more storage and more moving parts. For a network destination specifically, more moving parts means more that can go wrong; see Time Machine's own history with NAS destinations for a concrete example of what that costs in practice.
Which one do you actually need?
The right answer depends on the question you're actually asking:
- "What does my Documents folder look like right now, safely duplicated somewhere else?" — a mirror.
- "How do I keep two active locations consistent with each other?" — two-way sync.
- "How do I get back last Tuesday's version of a file I've since overwritten?" — a versioned backup.
- "Can my Mac boot from this drive if the internal one dies?" — a clone, and increasingly, the honest answer is: don't count on it.
Many people end up wanting a mirror and a versioned backup running side by side — that's not a contradiction, it's two different jobs. Time Machine (or Arq) covers "what did this look like before," and a plain mirror covers "give me an ordinary, current, browsable copy of the folders I actually care about, somewhere else." If you're building out a full strategy rather than picking a single tool, the 3-2-1 backup rule is the framework that ties mirror, versioned, and off-site copies together.
Questions
Is a mirror backup safe if I don't have version history?
For most files, yes — a mirror protects against the most common ways people actually lose data: drive failure, theft, or the source disappearing entirely. What it doesn't protect against is a mistake you don't catch before the next run — delete or overwrite a file at the source, and a plain mirror deletes or overwrites it at the destination too. Some tools soften this with a safety net (ThirdCopy can hold deleted files for a period instead of removing them immediately), but the honest fix for "I also want last Tuesday's version" is pairing a mirror with a versioned backup like Time Machine, not asking the mirror to become something it isn't.
What's the difference between sync and backup?
Sync — specifically two-way sync — keeps two active locations consistent with each other; both are meant to be worked from, and a change on either side updates the other. Backup implies a copy meant to be sat on and recovered from, not worked from directly, whether that's a one-way mirror (a current-state duplicate) or a versioned backup (historical states included). The practical tell: if you'd ever edit a file directly on your "backup" destination, you're actually syncing, not backing up.
Do I need a bootable clone in 2026?
Probably not, for most people. Apple Silicon's security model has made genuinely reliable bootable external clones impractical across the board — booting externally requires the internal drive's own recovery environment to bless it, which breaks down exactly when that internal drive is the thing that failed. If your Mac's internal drive dies, macOS's own reinstall-and-restore path, combined with a file-level backup and Migration Assistant, gets you working again more reliably than chasing a bootable clone does.
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See also: ThirdCopy vs. ChronoSync · ThirdCopy vs. FreeFileSync · the 3-2-1 backup rule for Mac users