I agree with what Laura Knotek and B. Diddy posted, once you opted to format your microSD card as Internal you can no longer use it as its own separate storage media. You have to selectively choose which option, Portable or Internal, and then it's a matter where you have to just go with the established rules that apply to each of them. No matter how many times you mount or dismount the card, those rules aren't going to change.
--- When you format the card to be Portable, this is its typical default. You can use the card to store files, transfer them between your phone's internal storage media and the card media, and whenever you need to dismount and remove it to use to transfer files to and from other devices (other mobile devices, computers, cameras, etc.). It can be used in other devices that are running different operating systems because the card is formatted as FAT32 or exFAT, two antiquated file systems that are limited in functionality and in reliability, but have the notable aspect in being supported by multiple platforms. It's this aspect you need to keep in mind as far as why you need to format the card as either Portable or as Internal.
-- When you format the card as Internal, this alters the card in several ways that can't be ignored. The card's file system gets changed to ext4 and it gets encrypted. Your phone's internal storage media is already set up to use ext4 and it's encrypted so formatting your card as Internal changes the card so it's now the same file system as your phone's internal storage. A really, really, really important you have to accept is when the card is set to be Internal, you should not remove the card. As far as the installed Android operating system on you phone is concerned, now the card is essentially a fixed component. The internal storage media and the microSD card storage media are virtually one storage media, now with added storage capacity.
It's your phone's OS that manages and maintains what does and doesn't get written to the card now. You don't interact with the card itself now, it's just part of the overall storage. This is also why you cannot use something like a file manager app to selectively pick and choose what to save to the card, it's no longer its own separate storage media volume -- now the card is a part of the total storage capacity. And one last point, since the card is now encrypted even if you do physically remove the card and try to mount it in a different device, that just won't work. The encryption prevents you being able to openly access anything that is residing in the card anyway. (and the encryption key that gets created ties that particular card to that particular phone)
So pick either Portable or Internal, but take into consideration the consequences if you pick Internal.
If you later change your mind, you can reformat your card back to Portable. This returns it back to a FAT file system and removes the encryption. But this is a somewhat significant change backwards so you might need to reinstall a few apps or there might be some settings/config options that you need to restore. But once the card is Portable again, then you can use it like before as a typical transfer storage media or whatever.
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