I moved my library and the attached external SSD off an Id to a PC. Now that I've changed the platform.pmd file all my new imports are showing the correct/Windows slash protocol.
So, question at this point - though everything works OK, I've got a library that a vast portion has a file location with the slashes going the wrong way when viewed in MC. I tried using Tools>Library Tools > Fill Properties from Filename but that didn't do the trick. Is there a way to do a mass change to correct this without losing other info like ratings, playlists, number of plays, etc?