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@Jack_Graham - to add to the posts form Toxaris and KevinH
The 'metadata.opf' files in calibre book folders serve one purpose only, as the means to restore the library database - 'metadata.db'. Apart from the Library Maintenance->Restore Database feature, the calibre Library Manager makes no use of these files. The calibre Viewer uses the content.opf file within the ePub in its metadata overlay display.
If you look inside a metadata.opf file (with an editor like Notepad++) you'll find it lacks the spine and manifest sections which are essential to an ePub (I think ?), You might also find data related to custom columns that is absent from the corresponding content.opf file contained within the ePub file.
It might have been better if the metadata.opf files in calibre library book folders were named metadata.bak, but with over 3 million active users it's too late to contemplate doing that now.
BR
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