@RomanZ: I checked the keys, and the master, on which the lightroom catalog was edited, has both keys (naturally), and the slave (Synology Diskstation) has only the RO key. Well, after the disaster I found only the .!sync files, and nothing else. Yes, 100% sure, there are only these two computers which have keys to this folder: one Mac (master) and one Synology (RO) sure, but it wasn't there. BTW: Lightroom saves the database several times while it's open, similar to Excel. Here is the current history from the Mac, running the current BT Sync 1.4: 2 hours ago Finished syncing with Schroeder 2 hours ago Finished syncing with Schroeder 2 hours ago Finished syncing with Schroeder 2 hours ago Finished syncing with Schroeder 2 hours ago Remote peer removed file Lightroom5.lrcat-journal 2 hours ago Remote peer added file Lightroom5.lrcat 2 hours ago Finished syncing with SALLY 2 hours ago Added file Lightroom5.lrcat-journal Schroeder is the Synology backup, which has only the RO key. SALLY is a third PC, but it syncs only other folders, not the Lightroom folder. What do the lines beginning with "Remote peer " mean exactly? The remote peer is Schroeder, having just the RO key, as I understand it?