Re: How to save camera roll photos to SD when using adoptive storage
Exactly what @N4Newbie said... in many ways you no longer have an SD card, it has been "adopted" by the system and is considered internal memory. The card can only be read in the phone and is mated to that device.
Best bet is to use Google Photos and setup auto backup (which everyone should be doing anyway), then if you need space you can delete the photos locally well still having access to them on your phone, PC, any Android device with your Google account, or anything with a web browser that can get to Google.