Android introduced the ability to use external storage as internal storage a few years ago, but this ties the SD card and phone together. If something goes wrong, you won’t be able to get back whatever data was on the card.
If you format the SD card or USB drive as internal storage, your phone’s original storage and the added storage merge into one giant pool. This means applications that don’t normally support saving data to external storage can take advantage of the extra space. It also means you don’t have to manually move apps back and forth between your phone’s storage and an SD card.
