New 27" Imac wont recongnize Canon camcorder or canon 50D camera when attached with USB

Hello, my friend.

You didn't say what version macOS you're running, but you say your iMac is new so I will assume 10.15.x Catalina.

There are a couple of things you can try right off.

Visit the App Store > Updates and see if there are any macOS updates that can be downloaded for your iMac.

If so, get those installed.

Visit the Canon website and look for driver or firmware updates for you cameras there. If something is available, download and install it.

Also, check the obvious... cables and connections... look for failures, frays, debris in the camera ports, etc.

Reboot the iMac and the cameras and try to make the connections with the cameras again.

If these things offer no resolution, let us know. We have lots of smart people with lots of great knowledge here.

Check the Canon Quick Start Guide and check to see if the camera(s) have a Disk Mode setting.

If not, you can remove the CF Card from the 50D and use a USB multi reader to import the images to your Mac.

Not sure which Canon camcorder you have?

If it has an SD card, you can put it into the iMac's SD card slot or use the USB multi card reader and import the video to your Mac.