iTunes not locating files

I have my iTunes media on an external hard drive (let’s call it external A) (I moved it there with the help of Apple support some years ago).

I have hundreds of playlists which I use for work and at vital.

I am trying to create a back up of it all.

I copied and pasted the iTunes folder from the external A over to a new external hard drive (B) but realised that when I tried to open iTunes from that that it didn’t have the most up to date library file.

it turns out for some reason the library file is being stored on the internal hard drive.

I copied and pasted that library file over to the new back up external B.

when I open iTunes and direct it external B it opens up the library and brings all my playlists in - which is what I need!

but… when I try to play any songs it can’t locate them. If I manually locate one it is fine and then it says it can locate the others in the same way but it doesn’t locate any.

I can’t manually locate thousands and thousands of songs.

I don’t know how to fix this?

I suspect part of the issue is that the iTunes folder is in a bit of a mess - I’ve used it for years, when I was on windows and then now on Mac. I have ‘consolidated’ and that hasn’t fixed it.

I’m not sure what to do?

Thanks

Nancy