Install udisks2 & udiskie
sudo pacman -S udisks2 udiskie
Create a systemd service that will start udiskie on startup.
[Unit]
Description=Removable disk automounter using udisks
[Service]
ExecStart=/usr/bin/udiskie -aN
[Install]
WantedBy=default.target
Save it to $HOME/.config/systemd/user/ so that it gets picked up by systemctl.
Enable it for current user & start it:
systemctl --user enable udiskie.service
systemctl --user start udiskie.service
systemctl --user status udiskie.service