Apparently external storage on Android (referred to by Environment.getExternalStorageDirectory) doesn't have to be an SD card. In fact, it doesn't even have to be external!
How can I refer to this storage in the UI? For example, if it is currently not accessible, I want to surface an error to the user. If I say "external storage" or "SD card", I may confuse users of some devices. Is there a way to appropriately identify it?