In case it matters, are you running dev or stable ?
If stable did you take the “OTA upgrade” route for stable builds outlined:
To confirm that the device is at least configured correctly for External storage please can you open Files app and open its navigation pane (tap hamburger menu top left). Now do you see the device listed lower half of the menu, showing its Free space then immediately below do you see the SD card and alongside it a working eject button ?
Correct behaviour is that tapping the Eject button will unmount the SD card and the device will continue to work nicely, simply without any External storage.
As I understand it, the R build has been constructed to update and work correctly / only with “SD card as External storage”.