Originally Posted by itimpi
For getting metadata embedded into the Calibre copy of ePub files by far the easiest way to do it is to make use of the Modify ePub plugin. Much easier than exporting the files and then re-importing them.
Thanks for the tip @itimpi. I was completely unaware of that plugin. Just shows it always pays to ask here before you do something major.
I've installed it and am just getting ready to play with it... no doc, but I assume you select some books and hit the button in the toolbar and it injects all the calibre metadata into the epub (or whatever format) ebook file. That's major goodness in that it makes partial restores or data base rebuilds easy I've always found calibre very reliable but I'm also a paranoid ex-banker who firmly believes in the power of Murphy.
Later this evening I'll backup my data base and then select all books and push the button and go to bed.