To mirror one folder to another, treat Folder A as the source of truth and make Folder B match it. That can involve copying new files, replacing changed files, and removing destination only items.
The deletion behavior is the part that separates a true mirror from a simple update, so the safest mirror workflow is comparison first, preview second, approval last.
Start with the broader guide to syncing two folders without installing software, and use the compare folders and copy changed files workflow to understand the difference list before approving removals.
What a Folder Mirror Actually Means
Suppose your source is:
Design Project/
and your destination is:
External Backup/Design Project/
If the source contains 500 files and the destination contains 520, a mirror does not simply add what is missing. It determines whether the 20 destination only files should remain.
If they are absent from the source and the destination is meant to be an exact copy, those extra files can be scheduled for removal.
The official FreeFileSync desktop manual describes Mirror in the same fundamental way: it creates and deletes target files as required until the target becomes an exact copy of the source.
When Mirror Is the Right Choice
Use mirror when:
- Folder A is authoritative.
- Folder B exists specifically as a matching replica.
- Source deletions should be reflected in the destination.
- You do not need destination only archive material.
- You can review and recover from a mistaken deletion if necessary.
Examples include a replaceable secondary copy of a working directory or a deployment staging folder that should match a master folder.
When Mirror Is the Wrong Choice
Avoid mirror when the destination is a cumulative archive.
If you intentionally remove old photos from the laptop but want them retained on an external drive, mirroring can defeat that goal.
Use Update Destination instead.
Also avoid automatic mirroring when both folders contain independent edits. A simple one way mirror can overwrite or remove useful work from the destination.
Step 1: Confirm Source and Destination
The most dangerous mirror error is reversing A and B.
Before scanning, confirm:
Folder A: authoritative source
Folder B: replaceable mirror destination
Display both full folder names or user visible paths clearly in the interface.
Step 2: Compare
A folder comparison should show:
- Source only
- Destination only
- Source newer
- Destination newer
- Different content
- Identical
Pay particular attention to destination only and destination newer items.
A destination newer file may represent work that never made it back to the source. If you overwrite it automatically, you may lose that version.
Step 3: Inspect Destination Deletions
Filter the preview to actions that remove destination files.
If the list includes something unexpected, stop and investigate.
For example, a large destination only count can mean:
- You selected the wrong source folder.
- A source drive failed to mount completely.
- A filter accidentally excluded content.
- Files were intentionally archived only in the destination.
- The folder structures do not align the way you thought.
A preview is a diagnostic tool, not merely a confirmation step.
Step 4: Keep a Recovery Copy for Important Data
A mirror is destructive by design when the destination contains files absent from the source.
Before a large first run, keep another copy of irreplaceable data. This is especially important when consolidating old backups or working with external drives that contain mixed purposes.
FreeFileSync.online does not claim operating system Recycle Bin integration or guaranteed recovery after removal.
Step 5: Choose the Comparison Method
Fast metadata comparison is appropriate when timestamps and sizes are reliable.
Exact content comparison is useful when you need strong content equality checks, but it requires more reading.
A practical mirror can compare metadata first and perform deeper checks where needed.
Step 6: Run and Verify
After approval, monitor progress and errors.
If copy verification is enabled, remember that verification adds additional file reading. It can confirm that the destination content matches the source read used for the check, but it does not replace a backup.
The desktop FreeFileSync documentation makes a similar distinction in its VerifyCopiedFiles setting.
Mirror Versus Update in One Example
Source A:
Photos/2026/
Destination B:
Photo Backup/
B also contains Photos/2025/.
If you mirror a higher level folder where Photos/2025/ has no source counterpart, the old directory may be considered extra.
If that history must stay, Update Destination is safer.
If the destination is intentionally disposable and should contain only the current source state, Mirror is appropriate.
Browser Based Mirror Limitations
The browser tool is not an unattended mirror service.
It does not run after the page closes, does not integrate with Task Scheduler, does not use Volume Shadow Copy, and does not claim to copy locked files, NTFS permissions, alternate data streams, or symbolic links as operating system objects.
Those limitations should shape which folders you choose.
Conclusion
The safest answer to how to mirror one folder to another is not “copy everything.” It is: define the source, compare the folders, inspect destination only items, keep recovery options, preview every removal, and then approve the mirror.
Use the Free Online File Sync tool with Mirror A to B only when Folder B is truly meant to match Folder A.