The problem is that formatting the card as internal storage fundamentally changes how the system saves data to the card -- so you won't be able to back up the current contents of the card and then copy them back to the card after you've formatted it as internal storage.
If you really need to format the card as internal storage, I suggest the following:
1. Backup all of the photos, videos, documents, etc. that you might have saved on the card to your hard drive (the easiest way would be to insert the card into your PC and use Windows File Explorer to copy the files.
2. Go through all of your app settings menus and look for an option to choose the default storage location. Change it to Internal/Phone Storage wherever possible. (Also, if there's a system settings option for this in the Storage menu, change it there as well.)
3. Don't worry about LOST.DIR -- that's only used for files that got lost or corrupted somehow during a glitch.
4. Format the card as internal storage.
5. Connect the phone to your PC via USB and copy all of those photos, videos, and documents back to the phone (if you still want them there).