External SD write protection
Starting with Android 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.
3C Toolbox provides an option to remove such protection on rooted device, in its Active Tweaks settings. It may or may not be available depending on your version of Android.
More information can be found here: