> you can enable debug diagnostics for devices by using the usbd ctl file.
Sorry, I wasn't aware of that. usbd(4) does describe setting environment variables in plan9.ini to change the arguments for usb drivers (I wasn't aware of that either), but doesn't mention that you can also do it dynamically via usbdctl. For example echo diskargs -d >/dev/usbdctl before plugging in the device (or -dd, -ddd etc for more verbosity). You may need to study the source of usb/disk to make sense of the output.
