The "complete fix is to format the card as external storage. "Internal" storage is a kind of Band-Aid - if the phone doesn't have enough internal storage (and eventually, all phones run out), adding an SD card as "internal storage" will help - for a short time. (When the card gets full, you can't just put in a larger one - you have to back up all your apps to something other than the phone, back up all their data, clear each app's data, uninstall each app, put in a larger card, then reinstall the app and restore its data. With enough apps, that can take a day or more. And it's a LOT of writing to the new card (writing is what eventually destroys a card.)
It's meant as a "fix" for a small phone (8GB or less internal storage as of 2016) - but it's not a real fix to a problem that can't be solved - a phone has a fixed amount of storage, so every phone will eventually run out, unless you don't add any apps or data. (SunOS does it the right way - storage is storage, regardless of where it is - or even WHAT it is - but formatting an SD card as internal storage is a kind-of/sort-of "fix" that doesn't really fix a problem that has nothing to do with the amount of storage in the phone.)