USB-Serial adapter not working on Pogoplug PRO

[bluechalk2]

I have a USB-Serial adapter that works on my Dockstar, but not in my PogoPlug PRO.

The dmesg output from the Dockstar is:
[2800494.201313] usb 1-1.1: new full speed USB device number 12 using orion-ehci
[2800494.317821] ch341 1-1.1:1.0: ch341-uart converter detected
[2800494.329857] usb 1-1.1: ch341-uart converter now attached to ttyUSB0

But, on the Pogoplug, I don't get a /dev/ttyUSB0. The dmesg says:
[ 6.740000] usb 1-1.1: new full speed USB device using oxnas-ehci and address 3
[ 6.850000] usb 1-1.1: New USB device found, idVendor=1a86, idProduct=7523
[ 6.850000] usb 1-1.1: New USB device strings: Mfr=0, Product=2, SerialNumber=0
[ 6.860000] usb 1-1.1: Product: USB2.0-Serial
[ 6.870000] usb 1-1.1: configuration #1 chosen from 1 choice

The Pogoplug does have the file
/lib/modules/3.1.4-2-ARCH/kernel/drivers/usb/serial/ch341.ko.gz

What am I missing?

[WarheadsSE]

Apparently, this post.