new user to Ultra SDXC UHS-I 128gb, need help how to copy files from pc

First time using an exfat 128Gb microSDXC card for my android, and I’m trying to copy media files from my Win10 pc, to no success. I’ve tried both connecting my phone in USB transfer mode, as well as directly through a card reader. PC can read the card, just can’t add files/folders. Any help appreciated.