systemd.mount not authorized

[Arno Gaboury <arnaud.gaboury@xxxxxxxxx>]

Dear list,

I boot with systemd and startx.

I can't open my USB disk with thunar (XFCE4 file manager) and get this
message:
Not autorized to perform operation.

I understand as a simple user I can not mount it. 
How can I solve this issue? I couldn't find any clear answer.

Thank you