You are strongly recommended to NOT store your calibre library on a NAS:
http://manual.calibre-ebook.com/faq....rked-drive-nas
On the other hand, using something like Microsoft SyncToy to mirror a copy onto your NAS is fine.
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Yes, calibre requires its own internal copies of the files it stores. Using links is fragile, it depends on files that calibre doesn't control and cannot guarantee will continue to exist.
I know iTunes allows you to use links, and I have had to clean up peoples' messes when they cannot find their music because it was e.g. downloaded into their %TEMP% downloads folder and got automatically cleaned.
