As root copy the steam automounter udev rules from:
/usr/lib/udev/rules.d/99-steamos-automount.rules
to:
/etc/udev/rules.d/99-steamos-automount.rules
Comment the two lines that begin
ACTION
by adding a
#
at the start of them i.e:
#ACTION=="add", RUN+="/usr/bin/systemd-run --no-block --collect /usr/libexec/hwsupport/block-device-event.sh add %k"
#ACTION=="remove", RUN+="/usr/bin/systemd-run --no-block --collect /usr/libexec/hwsupport/block-device-event.sh remove %k"
reload the udev rules:
udevadm control --reload