Why does Lightroom Change a Hardrive designation letter?

[michaelb68644745]

This is the second time this has happened. Lightroom manages to change the letter it uses to reach one of my hardrives, a letter that is not used on my computer. Then it moves all the files on that hardrive to the new letter. Then it can't find the photos because that designation only exists in Lightroom, not on my computer. For example Lightroom has invented a drive 'M' ( I do not have a drive M on my computer), moved the folders and files that are on my computer drive N to this fictitious drive M. The files have previously been imported into Lightroom from my computer drive N. Now Lightroom is looking for these files in drive M. But in reality there is no drive M. So Lightroom cannot find them. Last time this happened Lightroom lost over 40,000 photos and I had to start a new catalogue. This time it has lost 8.000 photos

All the data relating to the files is still held by Lightroom. I can 'find' a file, re import it and the post production is still there. I can find similar lost files at the same time, but only those in the same folder. I will take an age to 'find' all the photos by this method. Is there a way that I can persuade Lightroom to look in the correct folder instead of the invented one? And, why does Lighroom do this in the first place?

[ssprengel]

Are you sure the drive letter the photos were on hasn't just changed in Windows because you never assigned a permanent drive letter so Windows moves it around based on what else is plugged in?  That is the normal reason photos go missing, and you just need to assign a drive letter permanently.

In any case you shouldn't be re-importing the photos, you should just be telling LR where the photos are, now.  In LR, you should be able to right-click on the top-level folder(s) (on drive M in your example) and choose Find Missing Folder... and then point it to the correct location (on the N drive in your example) and LR should "move" things back within itself, for anything under the folder you chose. 

I have an example of missing folders because I have disconnected an external drive, so they'll show up when I plug the drive back into my computer and I don't need to go through the procedure I just described, but it lets me show you what the menu item looks like:

[Abambo]

Lr may have many faults and errors, but changing the drive letters is a Windows "feature" not something that Lr can manage or influence. Please see here for more information on how to fix this problem.

This, however, makes Lr apparently missing the photos, as Lr is not aware of the Windows drive letter changes. The data is still there, but you need to reestablish the connection. That is done as by ssprengel 's and these instructions: How to create and manage folders in Photoshop Lightroom Classic CC - Locate missing folders  .