I am using xfce 4.8 on Debian unstable. The xfdesktop's version is 4.8.2. I mount my hard disks/partitions through the fstab. Xfdesktop doesn't show them. However, they are shown in Nautilus or the gtk "open dialog". This same behaviour is seen in Thunar and xfce4-places-plugin. I will create the relevant bug reports and mention them here.
The Thunar bug-> https://bugzilla.xfce.org/show_bug.cgi?id=7667 The xfce4-places-plugin bug-> https://bugzilla.xfce.org/show_bug.cgi?id=7666
Closing, as xfdesktop has the same behavior as thunar/gio