How to get Google Photos to recognize External SD drive
Sorry, you can't do it the way you intend to. 


Google photos can back up to Ext.SD, only if the card is treated as internal storage. In your case the SD card is not treated as a part of device memory (portable) . 
Adoptable-storage is applicable only if the card is actually mounted on the device and not when connected via USB connector.


See adoptable-storage wiki 
for more details. 
See Google help which explains how the adaptable storage can be used for this purpose (read also the linked blog of Android pit)



What you can do, is to use apps like Folder Sync or  Auto Sync for Google drive and set up sync to your Ext.SD in a manner that the selected contents of your Drive are backed up on your SD and deleted from GDrive. This can be set up on schedule or on demand