The 1520 seems to do this on some apps (the equalizer, for instance). You go to the store, select the update, it downloads and won't install. When you click on the error it says that it must be installed to system memory not the card and gives you the option to override. Even if you say yes, the second attempt also results in an error...?
My 830 and sons 920 don't do this.
My 830 and sons 920 don't do this.