After upgrading to Xfce 4.14, my external USB drive does not stay spinned down anymore. Disabling volume management in Thunar does not fix it. The new xfce4-meta installed thunar-volman, which forces +udisks on gnome-base/gvfs and xfce-base/thunar, which is the cause of the problem, I think. After removing udisks flag from those 2 packages and uninstalling thunar-volman, my USB drive behaves as before.
If you have a problem with gvfs or volman, or whatever, file a bug there. The metapackage is not supposed to work as universal workaround for all problems. *** This bug has been marked as a duplicate of bug 698706 ***