Please make sure that you format your SD card as either FAT or ExFAT, as these are the only formats the app currently supports.
To change the storage location of your downloaded manga:
Step 1: Tap on icon to open the Manga Rock app’s side menu.
Step 2: Tap on icon to open Settings
Step 3: Under "Storage location", tap on Change to select download storage.
Step 4: In Select download storage screen, tap on the storage location of your choice.
You can also choose Manual Select to specify a storage folder.