How are you verifying that the GPS data disappeared? Which version of LR?
I selected all photos of the catalog. Then put a meta-data filter on "GPS data".
Which showed me that all photos where under "No coordinates".
I could not believe it.
What I did not check is wheter the photos themselves had the GPS data erased.
The first time this happened, was with LR 5 (I do not remember which version).
What I did not check is whether the photos themselves had the GPS data erased.
1. In the Import window, in the right panel under Apply During Import, make sure Metadata is set to None. It doesn't sound like this is your problem, but let's double check.
2. When it happens again, look at the Location tagset of the Metadata panel and click on several photos. Is the GPS data gone from there?
There are two cases
Case 1 : This happened under LR 5 or even LR 4 a couple of weeks or even months ago. The GPS data got erased everywhere and could not be recovered.
Case 2 : This happened under LR CC. I was lucky to be aware of that problem because shortly after this loss I tried to find photos for a specific location. But there was no GPS data available. By reloading the apparently uncorrupted catalog everything was fine again.
What I think happened
Since all GPS data for the entire catalog disappeared in one instant, I think that the problem lies in my catalog. There must be a file containing a link from the photos the GPS date (or the other way round). I think that link got corrupted or was erased.
In case 1 unaware of this loss I browsed and accessed the photos thereby gradually erasing the GPS data in the photos until none was left.
In case 2, since this is an unforeseen event for LR, the GPS data in the photos were not immediately erased. And I did not access the photos. By reloading an uncorrupted catalog everything was fine again.
Unfortunately I cannot trace what event caused the catalog to get corrupted. All I know is that I was creating and deleting collections.
Believe me, I now constantly check the presence of GPS data via the metadata filter "GPS data".
Does this make any sense to you ?
LR stores all of the catalog data in a SQL database, and given the technical nature of how the database stores its data, it's very unlikely an entire category of data (GPS) would get instantly deleted. So I think something else must have happened in case 1.
In case 2, it's possible that filtering by the GPS Data column stopped working, even though the GPS data for the individual photos remained intact. There have been a few reports over the years of filtering getting screwed up (but not for GPS, if I recall). So if your filter does start indicating missing GPS data, immediately check the Metadata panel, and see if the photos still show up on the Map.
Thanks for all your advice.
It is a mystery case.