I have used the GV K56flex PC card modem with my PB3400c for at least a year without problem, but upon upgrading to Mac OS 9 I ran into an incompatibility. At first I could not use the modem software at all without the Finder crashing, but after trashing the modem pref file, I found that the modem is stable as long as the GV PC Card control panel is not installed in the Control Panels folder. I have not tried to open it outside the folder as I have no need to access it. The only problem is that without the control panel installed, there is no graphic in the menu bar showing me the modem's activity. I can live with this until the software is updated, which I hope is in the works.