#1 NAS are not supported (in the FAQ). Things can go
south at any time.
I really suggest that
A NEW Main Library (or a copy of the target) be placed on a Local disc and Merge the rest into this one. Avoid the NASty in NAS
merging:

Use the Find Duplicates Plugin to get an idea of how many.
Preferences:Adding Books: controls how duplicates are handled when added. Make needed adjustments
select some books in SOURCE library: Right-click:Copy to Library: <name> (optional) Small batches suggested.