When you put in a SD card it should give you the option to set up as separate or internal storage. Set separate and it'll ask you what you wan to migrate over. You can also set from that point on where you w photos to be stored.
Insert a microSD card into your phone. Select Portable Storage as this will allow you to remove the SD card at any time and can be used in other devices. If you select Internal Storage, the card cannot be removed as it becomes part of your phones' internal storage. If removed and re-inserted, the card will be formatted when Portable storage is selected.
The Move Media dialog on card insertion will only be displayed once for each SD card, and only if there is content to move. You can invoke it manually from storage settings at any time by going to: Settings > Storage and USB > SD Card - tap three dots (menu) in the upper right corner of screen, tap Settings > Move Media.