My first clue was in this comment, I didn't have the usb_storage kernel module loaded:
# lsmod | grep usb usbhid 36882 0 hid 67742 1 usbhid
So I loaded the usb_storage module:
# modprobe usb_storage WARNING: All config files need .conf: /etc/modprobe.d/ndiswrapper, it will be ignored in a future release. # lsmod | grep usb usb_storage 40172 1 usbhid 36882 0 hid 67742 1 usbhid
And at this point USB drives mounted just fine. To make it permanent, I followed the directions in this post, added "usb_storage" to /etc/modules and rebooted. USB automounting working fine now.