Problems loading existing library into new Calibre installtion

[sdspieg]

I have a quite large collection of books (>100gb) that I manage through Calibre. After a problem with my boot-SSD, I had to reformat it and re-install Calibre. I pointed the Install wizard to the folder (on another drive) that still has my entire folder structure. The program then says that it will use an existing library automatically, but it did not do so. Instead it created a new metadata.db. Since I wanted to add some new books I bought, I did so - they are now in that library. But so what is the easiest way to get all other ones added to that metadata.db, based on the content in 100s of 'Calibre Library' folders? Thanks!