create a file /etc/polkit-1/localauthority/50-local.d/55-storage.pkla

with the content

[Storage Permissions]
Identity=unix-group:plugdev
Action=org.freedesktop.udisks.filesystem-mount;org.freedesktop.udisks.drive-eject;org.freedesktop.udisks.drive-detach;org.freedesktop.udisks.luks-unlock;org.freedesktop.udisks.inhibit-polling;org.freedesktop.udisks.drive-set-spindown
ResultAny=yes
ResultActive=yes
ResultInactive=no

more info