I haven't used my Prograf Pro-300 in a couple of months. I am running Windows 10 and have administrator privileges in my user account.
I went to print a B&W image from Photoshop, and set the print dialog to "Printer Manages Colors". I then went to "Print Settings" to open the printer driver. I went to set the paper type --- one of my non-Canon custom paper --- and find that none of my custom papers are listed in the Media Type in the Canon printer driver. My custom papers ARE properly stored in the printer (and of course they are in the Media Configuration Tool where I originally added them).
I went to the "Maintenance" tab on the printer driver and hit "Update Media Information" in hopes this would transfer the data from the printer to the printer driver. This opens the Update Media Information dialog box. I hit "Execute" and a window opens that reads "New media information that your driver can use is available" I hit OK, and the window closes - not much else happens and the media information is STILL not in the printer driver.
I think your best bet is to register your product and call Canon Support on Monday. I don’t see many Pro-300 users posting here.
12/05/2023: New firmware updates are available.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.