First, choose the relationship—not the button
A folder sync tool needs a rule for deciding which side wins. The right rule depends on whether one folder is authoritative, whether both sides can change, and whether deleting a file on one side should delete it on the other.
Update: add new and newer files
Update mode is the conservative starting point. New source files copy to the destination and newer source versions replace older destination versions. Destination-only files remain. It works well when you want to sync an external hard drive on Mac without turning the destination into an exact clone.
Mirror: make the destination match exactly
Mirror mode copies source additions and changes, then removes destination-only items. That is useful for replicas and publish folders, but it is destructive by definition. A safe mirror plan must show removals before execution and preserve the current destination content in a recovery checkpoint.
Two-way sync: reconcile edits on both sides
Two-way sync compares both folders against a prior baseline. If a file changed only on one side, that version can move to the other. If both sides changed, the tool should report a conflict instead of silently guessing. Without a baseline, modification dates alone are not enough to infer intent reliably.
A six-step workflow for reliable folder sync
- Connect both locations. Confirm external drives or network shares are fully mounted and writable.
- Select the mode. Choose Update, Mirror, or baseline-driven two-way behavior based on the relationship above.
- Scan both folders. Compare relative path, type, size, modification evidence, and content evidence where required.
- Review the plan. Look separately at copies, overwrites, conflicts, and removals. A total file count is not enough.
- Protect risky targets. Checkpoint the existing destination version before an overwrite or removal.
- Execute and verify. Verify copied bytes at the destination and keep a receipt describing the outcome.
What to inspect before approving
Pay special attention to unusually large transfer totals, unexpected top-level folders, a sudden wave of deletions, files changed on both sides, and paths that changed only by letter case. Those signals often reveal a wrong source, a newly mounted empty volume, an excluded folder, or a naming collision.
A good plan explains each action in plain language. “Copy Projects/brief.pdf because it exists only in the source” is reviewable. An unexplained queue of 2,000 operations is not.
External drives and network folders
Removable and network volumes can disconnect, become read-only, or report free space differently while a job is running. Use a tool that validates the selected volume again at execution time, writes through temporary sibling files, and refuses to run later destructive actions when an earlier prerequisite failed.
Cloud folders mounted by another provider may contain placeholders that are not downloaded yet. SyncReceipt treats the mounted folder as macOS presents it; the provider still controls availability. Test a small representative folder before relying on a new storage configuration.
How SyncReceipt approaches the job
SyncReceipt separates planning from execution. It presents stable reason codes for every action, performs content verification after copies, creates checkpoints before overwrites and Mirror deletions, and records the result in a self-verifying receipt. Free mode includes unlimited plan analysis and manual Update runs; Lifetime Pro adds Mirror, baseline-driven Two-Way execution, and JSON receipt export.
Verify the copy, not just the workflow
A completed progress bar only says the operation returned. For important folders, compare the actual destination bytes with a cryptographic digest after writing. Continue with the companion guide: how to verify copied files on Mac with SHA-256.
Keep a second backup
Folder sync is not the same as a versioned backup or a bootable disk image. A mistaken source edit can propagate through a correctly functioning sync. Keep an independent backup such as Time Machine, and periodically test that you can restore from it.