The Astro 320 is used for dog tracking purposes. I am not familiar with this model myself. Maybe another member can confirm the specifics of map files on a microSD for this model.

Assuming this model is capable of recognising multiple map files on a microSD, you need to place them either in a root folder named Garmin or in a root folder named Map (this is where I am not sure). Try either name. Then of course make sure you have unlocked the map files properly. The tutorial section may be your guide there.

What is the size of the individual map files?