I came across a very strange problem involving Vodafone’s K3520 (rebraded Huawei) 3G USB dongle. It simply refused to work on Ubuntu 10.10 on a certain laptop. While the USB storage part of it was recognized, the modem part was not.
Using the same K3520 on the same computer, with another operating system, used to work (before I installed Ubuntu on it).
First thing I checked was to make sure the USB dongle works – I plugged it into my netbook (Ubuntu 10.04) and on another laptop (Ubuntu 10.10, installed at the same time, identical setup as the non-working one). It worked on both machines.
I then used my own K3520 dongle to check the laptop with the problems. Mine didn’t work either. So it had to be a coflict between the hardware and Ubuntu 10.10.
After more than half an hour of poking around, loading/unloading modules, rebooting, googling and so on, I found out that the following commands make it work:
# rmmod usb-storage
# modprobe -r option
# modprobe -r usbserial
# modprobe usbserial vendor=0x12d1 product=0x1001
These need to be run with the dongle plugged in. The last command’s vendor and product parameters can be excluded, I think.
After trying these a couple of times so I could determine that the dongle was working correctly and was stable, I had to figure out how to convince udev to load the necessary modules and options every time the dongle is plugged in.
After digging through udev’s files and googling a lot, I found out that one line needs to be edited in /lib/udev/rules.d/40-usb_modeswitch.rules :
ATTRS{idVendor}=="12d1", ATTRS{idProduct}=="1001", RUN+="usb_modeswitch '%b/%k'"
needs to be changed to:
ATTRS{idVendor}=="12d1", ATTRS{idProduct}=="1001", RUN+="/sbin/modprobe option"
Don’t be confused by the fact that the comment above that line says it’s related to Huawei E169. E169, K3520, same thing, right? 
Plugging out the device and back in again should make it work forever and ever.
As for the reason why the dongle didn’t work on that machine with Ubuntu 10.10, while it worked on others with the same software setup… I have no idea.
Image credit: Julien Gong Min.
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