There are situations where a volume has been improperly ejected. When the volume is mounted again, the mount point of the previous time is still hanging around, and macOS cannot reuse it. As a result it will allocate a new mount point that has a slightly different name.
Since Peakto uses the path to access a catalog, changing the mount point will make the original path unavailable and the path will appear offline.
Let's take an example. Suppose our external drive is called "Media" and is improperly ejected.
When plugging it back, the original mount point '/Volumes/Media' is still there and empty. MacOS will allocate a new mount point '/Volumes/Media 1' and all the content of our "Media" drive will now have their path prefixed by '/Volumes/Media 1'.
Peakto will recognise that the path "/Volumes/Media" is still present and will mark the volume as online.
But any catalog with a path such as "/Volumes/Media/Catalog.photos" will not be accessible at that path, but rather at "/Volumes/Media 1/Catalog.photos".
The best way to get rid of this dangling mount point is to restart your Mac.