Basecamp can't see content of microSD card when GPS connected

I have 2 microSD cards, the first one contains OS 1/50,000 data, the second one OS 1/25,000 data. If I put the 1st one (1/50) in my GPS, connect it to laptop and open Basecamp, I can see the name of my OS file on the left hand side menu and the OS 1/50 data is well displayed on the map. Then I change microSD card in my GPS i.e. replace the OS 1/50 data by the OS 1/25, connect to laptop and open Basecamp, I still see the same data as before (1/50) on the left hand side menu and the maps displayed are still the OS 1/50. Why can't Basecamp see the OS 1/25 data? On both cards the .img file are under F:\Garmin.