External Storage, Expandable Memory, SD Cards

SD Cards are a cost effective alternative to expanding phone storage. The time a phone takes to read SD cards is slightly greater than other forms of memory, therefore, time-critical applications (like system software) are not installed on them. They are also more prone to damage over extended use and since they are swappable across devices, they can also get viruses and malwares.

If your phone has as SD card slot, then the best files to place on it could be media files, while avoiding critical documents and application data. Some mobile applications with large file sizes (like 3D games, offline GPS navigation systems etcetera) are developed considering this concept, storing important personalisation data on the phone's internal memory while keeping periodic gameplay data on the SD card and only pulling relevant segments to the internal memory when needed. In case the SD cards is lost or damaged, this data can be downloaded again without losing important user data.