Put this in /etc/udev/rules.d/10-automount.rules:
KERNEL!="sd[c-z][1-9]", GOTO="media_by_label_auto_mount_end"
# Global mount options
ACTION=="add", ENV{mount_options}="relatime,users,sync"
# Filesystem specific options
ACTION=="add", PROGRAM=="/lib/initcpio/udev/vol_id -t %N", RESULT=="vfat|ntfs", ENV{mount_options}="$env{mount_options},utf8,gid=100,umask=002"
ACTION=="add", PROGRAM=="/lib/initcpio/udev/vol_id --label %N", ENV{dir_name}="%c"
ACTION=="add", PROGRAM!="/lib/initcpio/udev/vol_id --label %N", ENV{dir_name}="usbhd-%k"
ACTION=="add", RUN+="/bin/mkdir -p /mnt/%E{dir_name}", RUN+="/bin/mount -o $env{mount_options} /dev/%k /mnt/%E{dir_name}"
ACTION=="remove", ENV{dir_name}=="?*", RUN+="/bin/umount -l /mnt/%E{dir_name}", RUN+="/bin/rmdir /mnt/%E{dir_name}"
LABEL="media_by_label_auto_mount_end"
You need to change the first line (specifically the [c-z] bit) if you
have more (or less) than two non-removable drives. I don't know exactly
how it works, but it does the job. I copied it from the arch wiki years
ago and I'm putting it here for my own reference.