how to fix?
Don't really know if there is a way to fix it but for me when my ssd was dieing the updater kept downloading the same files over nd over again, sorry but I can't really help you but it might just as much be just some freak thing.
If that is the case just delete the eir arts data file and the Exsternal one and it will want to redownload it, while you are at it you could just as much verify the game files on steam.
Also, did the computer crash with the game running inthe past weeks?