They're both hot swappable, but the one under the battery covery (the internal slot) is generally considered as the more pemanent of the two - the internal card is where the virtual memory swap file is stored, for example, and while the device works fine if the swap file disappears (because the card is removed) it's located on that card (and not the external one) since the internal card is considered less likely to be swapped in and out to load new music/video media etc.