How to get Google Photos to recognize External SD drive

I have connected an external SD card via a USB connector to my phone in order to have more space for recording video. However, Google Photos does not seem to recognize it - it does not show up in Device Folders, and I cannot figure out how to make it show up. In this [support thread](https://support.google.com/photos/thread/120725?msgid=121716) from last year it is stated that "if the device is running Android 6.0 and above then the external SD card may be merged into internal memory and in this case it will backup all of your photos including your SD card." I am running Android 9, and 9 > 6, so this should work. When I check if the SD card is merged with internal it is marked as "Portable storage," meaning that the SD card is set as external storage. However, it goes on to say that "you should have received a pop-up notification when the external SD card was detected. The notification gives you 2 options: Eject or Setup." The notification does not give me the option to "Setup", instead it only gives me the option to "Explore". That brings up some kind of file explorer from which I cannot find any options to merge storage. It goes in to say that you can "choose which folders on your phone to back up" by "Select Settings > Back up & sync. Touch "Choose folders to back up..." and select the folders you want to back up." But in the "Back up & sync" part of Google Photos, there is no "Choose folders to back up..." The closest I can see if "Back up device folders" which takes me to a list of already discovered folders, which does not include the SD card. How do I get Photos to recognize the external USB drive?