Risolto basta andare nel manifest e aggiungere:
SetApplicationAttribute(android:requestLegacyExternalStorage, true)
AddManifestText(<uses-permission
android:name="android.permission.WRITE_EXTERNAL_STORAGE"
android:maxSdkVersion="18" />)
poi:
rp.CheckAndRequest(rp.PERMISSION_WRITE_EXTERNAL_STORAGE)
Wait For Activity_PermissionResult (Permission As String, Result As Boolean)
If Result = True Then
'Creo la dir vuota per il file di log
File.MakeDir(File.Combine(File.DirRootExternal,"Download"),"test")
Dim Writer As TextWriter
'AppSettings.FileLogPath = (File.DirRootExternal,"Download"),"test")
'AppSettings.FileLogName = nome file
Writer.Initialize(File.OpenOutput(AppSettings.FileLogPath, AppSettings.FileLogName, True))
Writer.WriteLine(Text)
Writer.Close
end if