Let me explain me better :
I have a "Steam" partition, which is not mounted automatically at the startup.
If steam is started and I want to play a game which is on this hard drive, I have to mount it, then restart steam.
I am using Slackware Linux 64bits, with KDE and thunar as my file explorer.
Steam should watch dbus to detect newly mounted drives, or give us a mean to refresh the library.

dbus sample with my configuration :

signal sender=:1.87 -> dest=(null destination) serial=16 path=/org/gtk/Private/RemoteVolumeMonitor; interface=org.gtk.Private.RemoteVolumeMonitor; member=VolumeChanged
   string "org.gtk.Private.UDisks2VolumeMonitor"
   string "0xd1e5b0"
   struct {
      string "0xd1e5b0"
      string "Steam"
      string ". GThemedIcon drive-harddisk drive"
      string ". GThemedIcon drive-harddisk-symbolic drive-harddisk drive"
      string ""
      string ""
      boolean true
      boolean false
      string "0xd38ab0"
      string "0xd96940"
      array [
         dict entry(
            string "class"
            string "device"
         )
         dict entry(
            string "unix-device"
            string "/dev/sda2"
         )
         dict entry(
            string "label"
            string "Steam"
         )
         dict entry(
            string "uuid"
            string "5eeb8aa9-c284-4297-88e4-1f0c37f94f81"
         )
      ]
      string "gvfs.time_detected_usec.1385667152581259"
      array [
      ]
   }
signal sender=:1.87 -> dest=(null destination) serial=17 path=/org/gtk/Private/RemoteVolumeMonitor; interface=org.gtk.Private.RemoteVolumeMonitor; member=MountAdded
   string "org.gtk.Private.UDisks2VolumeMonitor"
   string "0xd96940"
   struct {
      string "0xd96940"
      string "Steam"
      string ". GThemedIcon drive-harddisk drive"
      string ". GThemedIcon drive-harddisk-symbolic drive-harddisk drive"
      string ""
      string "file:///run/media/mayeul/Steam"
      boolean true
      string "0xd1e5b0"
      array [
      ]
      string "gvfs.time_detected_usec.1385667206776064"
      array [
      ]
   }

I am not exactly a dbus expert, so I let this to your appreciation ;-)

Thank you for your already amazing work :-)