The portable library

Not a backup you restore — a library you open

A backup gives your files back after something has gone wrong. This gives you the library itself: the same grid, the same ratings, the same albums, on a laptop in a hotel room or on a machine at the other end of the country. FastLoupe calls it the Ready sync library. You choose the drive in Library ▾ → Settings… → Ready sync folder, and you watch it from Library ▾ → Library health….

Passes run by themselves once the drive is plugged in and you have stepped away from the keyboard for a couple of minutes. A drive that is not connected is a calm nothing to do, not a failure — a travel drive spends most of its life in a drawer, and there is no dialog anywhere in this feature.

What lands on the drive

The folder you choose is the library root. Nothing is inserted in front of a photograph's position under it, so a drive that already holds a plain copy of your pictures is adopted rather than duplicated.

L:\Pictures\2024\…                          your photos, at the same position as at home
L:\Pictures\2024\a.arw.xmp                  sidecars travel with their photo
L:\Pictures\zappic.db                       the catalogue, anchored to the drive it sits on
L:\Pictures\cache\thumbs\                    the previews, where the app already looks for them
L:\Pictures\Open FastLoupe library.cmd      the Windows launcher
L:\Pictures\Open FastLoupe library.command  the macOS launcher

A handful of top-level names belong to the clone: cache, faces, zappic.db, the staging folder and the two launchers. If your library root already contains one of them, the copy is refused by name before a single byte moves rather than mixing your folders into the clone's own.

It opens by double-click, on either computer

Every pass writes both launchers, whichever machine ran it. A drive that travels between Windows and a Mac is the entire point, so a clone carrying only the launcher of the computer that wrote it would open on half the machines it was made for.

They are plain text you can read before you double-click: a comment saying what the file is, and one command. Nothing is downloaded and nothing is hidden. Neither of them records where the drive is, either — each works out the library's location from its own position on the disk, so the clone keeps working whatever drive letter Windows gives it and wherever macOS mounts it.

The macOS edition is on the Mac App Store now. The Mac launcher is written on every pass, so a drive made today opens on either side — and the clone is still a perfectly good Windows-to-Windows copy.

Incremental, with an estimate you can trust

The first pass copies everything. Every pass after it copies what changed, compared on size and modification time — the same test robocopy and rsync use. Re-reading half a terabyte on every pass to discover that nothing changed is not a background job; it is a disk-thrashing one that people switch off within a week. The honest cost of that choice: a file another program rewrote to exactly the same byte count within the same two seconds is not noticed.

While a pass runs, the card names the phase it is actually in — looking for what has changed, copying photos and videos, copying previews, finishing — and draws its bar from bytes rather than file counts, because a set running from a 2 KB sidecar to a 60 MB raw makes a file count crawl and then leap. The remaining-time estimate appears only once the measured rate has settled, and withdraws itself if the drive stalls. An estimate that swings is worse than none.

A stalled drive does not end the pass

Hours of sustained writing over USB will meet a transient sooner or later. A file that fails that way is retried in place — three attempts, waiting one second, then five, then fifteen — and the readout says so rather than showing a count beside a bar that has not moved, which is indistinguishable from the hang this exists to survive. A file that fails for a real reason — permission, no space, the source gone — is recorded straight away with no waiting at all.

Either way the pass carries on and the clone is still published. The files that were skipped are counted, listed, and retried by the next pass on their own. There is no button to press. Only a destination that has actually gone away stops a pass.

Nothing on that drive is ever deleted

Not an extra file, not a folder, not a photograph the library no longer has. Files at the destination that are not part of the library are counted, up to fifty of them are named, and they are left exactly where they are.

The clone exists to be a second copy, and a mirror that deletes in order to converge is one mistaken root away from deleting the only copy of something. The cost is that the drive accumulates — a photograph you removed from the library stays there until you remove it yourself — and that is the right way round.

Format the drive exFAT

exFAT is the one format Windows and macOS can both write. If the drive is NTFS the copy is still a perfectly good Windows drive, but a Mac can only read it, so the library opens there for browsing only. Everything that reads works — the grid, the loupe, search, filters, compare, the slideshow, the second screen, the map, duplicates — and everything that writes is off.

FastLoupe says so in a strip at the top of the window from the moment the library opens, not at the first refused keystroke, and the strip stays for the whole session. Learning this from a rating that would not stick, an hour in, is the failure it exists to prevent.

Where you choose the destination, FastLoupe also tells you if the volume you picked is one macOS mounts read-only, and offers the reformatting instructions — with the erase warning and the honest cost of the rebuild first, because reformatting means copying everything again. It is advice, not a refusal.

What it does not do

FastLoupe --ready-sync L:\Pictures