FreeFileSync

FreeFileSync compares two folders and then makes one of them look like the other, or makes both agree, according to a rule you choose. That sounds like a small job, and for a single pair of folders it is. What has earned the program a following is everything around it: it shows precisely which files will be created, overwritten or deleted before it touches anything, it saves a whole configuration as a reusable batch job, and it can watch folders and run that job whenever something changes. It is free software under the GNU GPL version 3 or later and runs on Windows, macOS and Linux.

Compare first, then synchronize

The workflow is deliberately two-step. You pick the left and right folders, choose how the comparison should decide whether two files are the same, and press Compare. The result is a colour-coded list of planned actions with the amount of work summarised. Only after reading it do you press Synchronize. The comparison method matters more than it looks: comparing modification time and size is fast but can be misled by clock differences or by a copy that changed a timestamp, while comparing content is exact and has to read both sides, which is slow over a network share. A third option compares size only, which is quick and occasionally useful, and occasionally wrong.

Sync variants, filters and versioning

Four variants define the behaviour. Mirror makes the target identical to the source and therefore deletes whatever is missing there. Update copies new and changed files but never deletes anything. Two-way propagates changes in both directions and reports conflicts for a human to resolve. Custom lets each direction be defined separately. Filters restrict a job to particular paths, names or file types, which matters as soon as one folder holds both documents and caches. Versioning is the safety net that makes Mirror defensible: instead of deleting, files can be moved to the recycle bin or to a timestamped folder beside the target, so a deletion can be undone. When a run hits an error, the failure is listed with a retry option rather than being skipped quietly.

Batch jobs and real-time triggers

A configured job can be saved and run again, with or without prompts, which is what makes unattended backups possible from Task Scheduler or a scheduled script. RealTimeSync watches one or more directories and starts a saved batch job shortly after changes stop arriving. It is a trigger, not a continuous replication engine: the job still compares and copies folder to folder, so the same rules and the same versioning apply. The project also publishes a portable build, and some transports and conveniences are reserved for the paid Donation Edition, so the official comparison is the place to check whether a feature you need is in the free build. Downloading only from the official site is worth insisting on, since mirror sites and bundled download managers are a familiar route for unwanted extras.

Limits, and where another tool is better

FreeFileSync is a folder synchronizer, not a backup system with history. A sync is supposed to propagate deletions and mistakes to the other side, and files encrypted by ransomware on one side will be faithfully mirrored to the other, which is why versioning and a genuinely independent backup both matter. Large trees take real time to compare, long paths and permission errors surface as failures that need attention, and a two-way sync over continuously changing data will generate conflicts that only a person can settle. If you need snapshots and restore points stretching back weeks, use a versioned backup tool. If you need block-level replication of a running system or a virtual machine, use a product built for that. If you want a particular cloud provider's photos synchronised, its own client will do it with less configuration. For keeping two folders that should agree actually agreeing, with your eyes open before anything is written, this is the tool.