There are several firmware releases for the M601/M602/M603 series of printers that have trouble successfully installing the Barcodejet module. If you are installing on these printers, it is recommended that you update the firmware on the printer to the latest revision. Take the following steps:
1) Power down printer and remove the USB device
2) Update the printer firmware per HP's instructions here
3) Power down printer and re-install the USB device
4) Power up printer, wait for it to come to ready, and reprint the Barcodejet test page.