"Unable to open USB device"

[Anonymous]

I found the problem, and can now clearly identify a serious bug in CUPS:

>
>  Printer State: idle, accepting jobs.
> Device URI: usb:/dev/usb/lp0
>
> but as soon as I try to print a test page, it says:
>
>  Printer State: stopped, accepting jobs.
> "Unable to open USB device "usb:/dev/usb/lp0": No such device"
> Device URI: usb:/dev/usb/lp0

What had happened was that when getting CUPS to work, I'd loaded usblp.ko (the usb printer driver) manually with modprobe, and forgotten to put it in the startup. So that after I rebooted, there was no usblp.ko present.

However, CUPS diverted me from this simple problem by its message, reproduced above, "Printer State: idle, accepting jobs.
> Device URI: usb:/dev/usb/lp0"

But THERE'S NO DRIVER!! Windows can tell when a local printer doesn't have a driver loaded, so there is no technical reason why CUPS should not. A correct diagnostic when starting the printer, instead of the above "idle, accepting jobs" nonsense, would have saved me a lot of time (hours).