Unfortunately my internal storage (32GB) is running out and I'd like to use the SD card (128GB) to add to internal storage. I understand that Samsung have made this difficult but not impossible? Would anybody be able to point me to online step-by-step guides/resources to help me overcome this limitation. Thanks for your help.
Well, if the option was restricted by the manufacturer there is no way you can alter that without altering the device (which can then brick a device so I don't recommend that stuff). However, you can try the instructions below and see if it will work for you. If not, it is restricted.
First, you need to be using a tablet that is running at least Android "Marshmallow" (or newer) or be able to upgrade to it. Second, you will need the instructions I am including below. And third, you need a phone that has not been restricted by the manufacturer from making this change. Which does happen.
NOTE: If any of the steps are missing on your tablet, or they don't work for you, then you have a tablet that either has an older Android version or has been restricted by the manufacturer.
How to turn the "External" SD card into "Internal" storage on "Marshmallow".
NOTE: Please be sure you start with a blank SD card. It needs to be blank because when the card is converted to "Internal" storage, it will be formatted (wiped) and encoded (making the cards data only readable by that device).
1. Go to device “Settings”, then select “Storage”.
2. Select your "SD Card", then tap the “three-dot menu“ (top-right), now select “Settings” from in there.
3. Now select “Format as internal”, and then “Erase & Format”.
4. Your SD Card will now be formatted as internal storage.
5. Reboot your tablet.
NOTE: If you don't reboot the tablet, many things may not work correctly, so make sure you do.
How to turn the "External" SD card into "Internal" storage on "Nougat".
IMPORTANT NOTE: Be sure you start with a blank SD card. When the card is converted to "Internal" storage, it will be formatted (wiped) and encoded (making the cards data only readable by that device), so anything on it prior would be gone.
1. Go to "Settings", and then select "Storage & USB".
2. At the bottom of the list you should see the SD card's details, including the option to format it and make it "Internal" storage.
3. Once this is done, reboot the device and you can start running things from the card.
If you don't reboot the tablet, some things may not work correctly, so make sure you do the reboot.