Tracks disappearing from engine

Having a strange issue with Engine on Mac. Whilst open, suddenly most of the tracks disappear from the collection! This time I was able to restart and they all came back, but before I had to delete the engine library and replace it with the one I luckily had recently updated on the USB. Anyone else having this issue?

Now when I update the library on the USB drive, instead of simply adding the new songs it takes 10 minutes to presumably copy the whole library over, despite them being essentially the same!