I'm writing files from a scanner to a Watch Folder in Lightroom. I've done this numerous times without issue.
Now, Lightroom tries to read a file that is still open and errors, popping up a model dialog box that says "Unable to read file", and just stops trying.
Is this something new in Lightroom CC? It never happened in LR5 or LR6. I'm guessing Lightroom would wait for the file to be fully written?
LR does have some issues with sharing files with other applications. LR5 did a better job of this than LR6/cc does.
My recommendation would be to not have LR running while you are scanning the images.