Chaley: I am now confused - not an uncommon state
As you mentioned you were running in "Narrow" mode I tried switching Calibre to that mode - and then drag-and-drop of book files to the Book Details pain worked fine.
I then switched back to my standard "Wide" configuration - and found it was now working in that mode as well. I do not know why yesterdaqy I was getting a no-entry symbiol
Maybe switching the modes reset something ?
Now that this is working it will be a great time-save for me when updating existing book entries with new files
I am surprised that updating a format this way has not been mentioned in other forum posts - I have always seen opening the Edit Metadata dialog mentioned as the recommeneded way to do this.