USB devices fail to automount unless Thunar is running

[MR Zenwiz]

I'm running Xubuntu 14.04.3:

 3.13.0-57-generic #95-Ubuntu SMP Fri Jun 19 09:28:15 UTC 2015 x86_64 GNU/Linux
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=14.04
DISTRIB_CODENAME=trusty
DISTRIB_DESCRIPTION="Ubuntu 14.04.3 LTS"

thunar:
  Installed: 1.6.3-1ubuntu5
  Candidate: 1.6.3-1ubuntu5
  Version table:
 *** 1.6.3-1ubuntu5 0
        500 http://us.archive.ubuntu.com/ubuntu/ trusty/universe amd64 Packages
        100 /var/lib/dpkg/status

with all the latest updates. I run this on three different machines - my home desktop, my work desktop and my home laptop.

As of about two weeks ago, I upgraded my home desktop to an Intel base, and last week it stopped automounting any USB disk/flash drives UNLESS Thunar is also running when I plug the drives in. (My work desktop is also an Intel base and this never happens there.)

This happens for my usual flash drives and also my SATA-USB3 conversion plug-in tray.

When the drive plugs in, it shows up in lsusb, but the notification never appears, df does not see it and I can't access it normally.

I discovered that, if I am not running Thunar when the drive is plugged in, I can run it, then click on the drive icon in the left vertical panel, and that mounts the drive.

If I am running Thunar, it mounts (semi)automatically with full notification, just like it used to without Thunar..

I have checked the Removable Drives and Media panel of settings, and the Mount removable drives when hot plugged and Mount removable media when inserted boxes are both checked.

What I expected to happen:

The drive mounts and is visible in df and accessible for all normal commands.

What happened was:

The drive shows up in lsusb but nowhere else unless Thunar is running or I start it up and click on the drive name.