Boot from ssd and use sd card as backup target

I mirgated to ssd for my HA running HA OS on RaspP4. How can I format as SD to use it as backup target?

The advantage would be that I can have the latest backup on a separate media, and have less need to frequently download it to another device.

After booting from ssd and inserting a FAT32 formatted sd card, I still only see sda under /dev. How can I mount (preferably auto-mount) my sd card?