Mirror can delete destination files to match the source. Verify the folder roles and proposed actions before running the job.
Mirror has a specific meaning
The FreeFileSync manual defines the left folder as the source and the right as the target for Mirror. It can create and delete target files until that side matches the source. That is useful for some workflows, but it should not be confused with retaining every historical version of a file.
Read the paths, not just the labels
Our suggested check is to read both full folder paths and identify which side contains the work you intend to preserve. Check connected drive names carefully. If you cannot explain why a proposed destination deletion is expected, pause before running the synchronization. A familiar folder name is not enough when several disks have similar structures.
Test the rule on disposable material
Make two sample folders and compare a deliberately different file in each. Inspect the proposed copy and deletion actions before executing that small test. The manual distinguishes Mirror, Update and Two way, so choose based on your intention rather than treating the variants as interchangeable. Do not use important originals as test material.
Synchronization is not a recovery test
After an intended sync, inspect the destination and retain a separate recovery copy where appropriate. A current mirror may reflect an unwanted source change just as faithfully as a wanted one. Version retention and restoration checks are different tasks. This guide does not run a job, access personal folders or certify an untested backup.
Go straight to the people who make it.
Mirror, Update and Two way definitionsSource check: 2026-10-02. General guidance; follow your device’s current instructions.