Hi! Thank you for contributing to the Canon Community! I see that you are getting a message saying your PRO-300 cannot be detected when trying to update the firmware.
First, we recommend that you check the printer's current firmware version. You can do that on the printer's LCD display by selecting "Various Settings" from the main menu. Then, select "Printer Settings" and choose "Firmware Update." After that, select "Check Current Version." If the current version is 1.010 or later (such as version 2.010), you do not need to update the printer's firmware.
If the printer's current firmware is something earlier than 1.010, then you will need to use your computer to update the firmware. You will also need a proper USB cable. The Firmware Updater will not be able to perform the update over a wireless connection, so you will get a message saying the printer cannot be detected if the cable is not connected.
I also recommend reading the installation instructions on the download site. Here is a link that will open that page:
https://www.usa.canon.com/support/p/imageprograf-pro-300
When the page opens, you can scroll down and click on Firmware. Then, click on the "Details" link under the file named "Printer Firmware Updater (Mac) for the PRO-300 Ver.1.0." From there, you can read the "File Description," before clicking on the "Installation Instruction" link below the description.
If you have been following the installation instructions mentioned on the website, I recommend checking the USB cable. Please note that we do not recommend using an adapter or an external USB hub. The cable needs to be connected directly to a port on your computer.
While I have heard of customers having better luck with an adapter made by Apple, we recommend using a USB cable (6 feet long maximum) that has the proper fittings on both ends. One end of the cable will need to have a square (USB-B) connector. That end of the cable connects to the square USB port on the back of the printer.
The other end of the cable should fit the port that your computer has. Most Mac computers have oval-shaped (USB-C) ports. If this is the type of port that your computer has, the other end of the cable needs to have an oval (USB-C) connector. If you don't have a USB cable with USB-B and USB-C connectors, you can purchase one online or from stores that sell computers and printers.
If you have an older Mac, the USB port might be rectangular-shaped. If that is the case, you need a cable that has USB-B connector on one end and a rectangular (USB-A) port on the other end. Please make sure that the cable is no longer than 6 feet (maximum length).
If the cable has the proper connectors on both ends, you will not need an adapter or external USB-hub. The cable can be connected directly to the USB port on your computer. If you are still having trouble, then a security program or computer setting might be interfering with the computer's ability to detect and communicate with the printer. I recommend contacting the security software developer or Apple for help, or you can try using a different computer to update the printer's firmware.
The most current version of the printer's firmware is 2.020. If your printer's firmware is already 1.010 or newer and you want to update to 2.020, you can do that through the options on your printer's LCD screen. Here is a web article that will help with that:
https://support.usa.canon.com/kb/s/article/ART180179
I hope this information helps. Thanks again.
