Calibre's library is meant to be the place where you store your books.
I'm curious why you want to store a second copy outside Calibre. Is it because:
1. Calibre doesn't allow you to find books the way you want? (e.g. tagging doesn't have the all metadata you want)
2. You have other software (e.g. ebook reader software) that needs to read ebook files, and it's impossible/inconvenient to use Calibre's OPDS server for that purpose?
No, there is not, and never will be:
https://manual.calibre-ebook.com/faq.html#why-doesn-t-calibr...
Calibre's library is meant to be the place where you store your books.
I'm curious why you want to store a second copy outside Calibre. Is it because:
1. Calibre doesn't allow you to find books the way you want? (e.g. tagging doesn't have the all metadata you want)
2. You have other software (e.g. ebook reader software) that needs to read ebook files, and it's impossible/inconvenient to use Calibre's OPDS server for that purpose?
3. Some other reason?