While the drive is currently unmounted (and preferably from a clean reboot):
sudo mount -v -o rw,x-mount.mkdir /dev/sdd1 /run/media/username/CORE
Replace sdd1, username, and CORE with what you use on your system.
Then check if it’s still being mounted as read-only (ro):
mount | grep sdd1
Then check current messages for any hints as to why it’s mounting it as read-only, using Up Arrow and Down Arrow to navigate if necessary. Press “q” to exit the log viewer:
journalctl -k -g mount
You can then umount and remove the temporary directory when you’re done:
sudo umount /dev/sdd1
sudo rmdir /run/media/username/CORE