On Android 5.0.1 SP work from external SD:

This path don't work for external SD: /storage/extSdCard/Garmin

This path work for external SD: /storage/extSdCard/Android/data/com.garmin.android.apps.gmobilext/files/Garmin/

You must add Garmin folder in Android/data/com.garmin.android.apps.gmobilext/files/ folder.

After install SP apk turn phone off/on and all will work like charm on your phone from external SD