"You'll lose the thumbnails in the libraries, but, as you say, you can recreate them fairly easily." It's generally only the geometries .rsrs that get corrupted. I don't recall ever having one of the preview ones go bad, but true you can delete them. To prevent the problem, just change the attributes of the files to read only. Of course, you have to remember to do it to newly added files. From a (on Windows) command prompt in the geometries folder the command: attrib +R *.rsr /s will set it for all rsr files in all subdirectories. You should only need to delete one if you modify the .obj file, for instance changing the UV mapping.

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