- Restore photos from your backups; if you don't have backups (why not?) then go to option 2
- Use an "undelete utility" to see if you can recover the files from your hard drive
This is a rare but recurring problem with Lightroom, despite Adobe's adamant denial that they cannot find any such thing happening after intensive investigation
In the future, you would be wise to not move photos inside of Lightroom. Instead, you could move the photos in your operating system and then reconnect in Lightroom via Adobe Lightroom - Find moved or missing files and folders; or better yet, develop a workflow that does not require moving photos from here to there (problem solved!)
Thanks for your quick reply. I already restored the files from backup now. Usually my workflow prevents me from having to move files again - however I had to correct a date which in return made me move the files to another folder. Anyway, thanks, I will do that system-wise now, if I have to.
I was just hoping to find out what causes this issue - but according to the other threads there is no consensus on that. I was afraid that my slightly unusual combination of Yosemite with Paragon NTFS and the photos on an external USB3-HDD with NTFS caused this behavior - but since this seems to be a common problem, probably not...