SD card not appearing when formatted as internal
When you format your SD card as internal storage (aka adoptable-storage), it will be encrypted. The key for en/decryption is stored on the device only. Hence all that a PC can see is "there is a partition but I don't know what's on it": as it hasn't access to the key, it cannot decrypt the contents.

For further details, see e.g. our adoptable-storage tag-wiki and follow the links from there.