Embed Metadata plugin tool creating errors
I recently started to use the 'Embed Metadata' plugin tool. It's not something I've felt any great need for in the past but my experiments with EPUB2 to EPUB3 format-shifting made me realise that it's much easier to do if calibre has made the first pass at sanitising the original OPF file.
However, I discovered that a small number of my EPUB2s had zero Check Book errors before Embed Metadata but a level 3 error immediately afterwards, 'The OPF has no unique identifier'
I tracked down the problem to a combination of 2 things:
1. The original content of the OPF main <dc:identifier>, i.e. the one whose 'id' attrib matches the <package> 'unique-identifier' attrib.
2. My calibre library's 'identifiers' field was empty. This field is not one I've previously paid much attention to.
Both conditions had to be met to trigger the error. For the problematic books, if I added an ISBN value to the library before running Embed Metadata, then no new errors were created.
I've sorted out my own library but I've attached a couple of problem EPUBs (scrambled) in case you want to investigate further.