On a new phone with Android One/10 I mounted a 256GB sd card. I formatted is and added it to the system as internal memory (device storage), which is 128GB. Now the system reports a 512GB SD card. I let the system move data to the sd card, and that data is now gone. I can see thumbnails of photos and videos, but can't open or play them. I have a backup, so can restore, but I need to know what is happening and find a way to undo this mess.
After unmounting the card I cannot make photos anymore. It reports too little memory, with about 100GB free on the internal storage. Reformatting the sd card results in a message to choose what to do with it. Choosing external storage and formatting gave the 256GB card back, but internal storage (/sdcard) is gone. In Total Commander, I cannot open it and get an error. The camera app won't open anymore. Evernote refuses to work. Etc etc...
There are hundreds of websites with instructions on how to add internal storage, but none about undoing it.
How can I get the system to use the device storage again?