Hi,
Use ☰ -> Settings -> Library -> Choose library folders and delete ghost SDcard.

I got a larger SD card for my phone and moved my music from one to the other using my computer.

Even though the location of the music is technically the same, MM imported all the songs again, but left the old one in the library. Now every song (over 20k) has a usable version and a ghost.

How do I remove the ghosts?

Preferably without having to select each and deleting it.