External SD write protection
Starting with Android 4.2/4.3, apps granted READ/WRITE permission to device's storage do not actually get those permissions on any external storage, be it an SD card or an USB OTG ! Those permissions only apply to internal storage.
Starting with Android 5.1, apps can acquire write permission through SAF (Simple Android Filesystem), which requires user to manually grant access from a specific folder (or root) of each external storage.
More information can be found here: