You just have to go back into the settings and change it back from internal. (in settings and storage).
And no other device will read the card as it is on the device you did the change through. The card becomes encoded, so that it cannot be accessed by other devices. It is a security measure so that someone can't just remove the SD card and take all the data. The card can be reused, but it would have to be formatted first. Which would erase all that is on it. The only way to get the data off of it first is to keep it in the device that the change was made in and then change it from "Internal" to "External" storage again. Note: Before doing that you would need to move the data from the card back onto the device. If you don't, you would...