Same issue here. I am having a download error while downloading any file in SD card through Chrome. Similar issues exist with annotating or editing a PDF which is in SD card. It is read only. Even though my memory card also belongs to Samsung.
When I talked to Samsung they said that this is a feature. Lol! And they won't fix it. You have limited permission of editing and writing on memory card in samsung devices and samsung memory card because of trust issues with memory card.