I tried Cyanogenmod 11 and tried to edit /system/etc/permissions/platform.xml because everybody advice to add group gid="media_rw" to WRITE_EXTERNAL_STORAGE to make SD card writable. So I thought that reverse action (i.e. removing the groups) will make it read-only. But there is nothing to remove .. it doesn't contain this group.
So I tried to change the permissions directly but SD cards use FAT and permissions cannot be changed.
Could you please help me? I would appreciate any advice. I really would like to have a device where I can test this stupid Google's restriction. Thank you !