Thank you. What I called "~" files are always .!Sync files created by Sync itself, so locking by other applications shouldn't be a problem. (There is no concurrent software like Dropbox, to anticipate an obvious question). These get usually created and destroyed in the normal sync process, but every so often it seems Sync starts to think that one of these files is not its own temporary file, but it's a legit file that needs to be synced. I might be wrong though. I agree that stopping Sync on all my devices at once and deleting all !Sync files would work, but that would require taking my work computer home, or viceversa, which is not something I can do on a sustainable basis. Also, I am not very familiar with the use of .SyncIgnore, but am I right to think that I need to set that separately in every directory? Is there a way to ask Sync to ignore all files starting with "~" by default? Thanks again and best, Matt