This happened to me at least once before.
What happens is, that after updating and rebooting (Fedora 30), xfdesktop and Thunar don't show removable drives anymore. Sometimes this issue seems to fix itself after rebooting, but not always.
To mount drives I usually use
gnome-disks in those situations. When doing so, they appear as non-removable disks in Thunar, and are still not showing up on the desktop.
killall xfdesktop and
xfdesktop --reload both don't help.
~/.config/xfce4-session/ is empty.
This doesn't seem to be an issue with a specific version of xfdesktop or Thunar, though. I have a different machine which runs the same OS and the same DE, yet, even when updating packages at the same time, to the same version, one shows the error while the other doesn't.