Connecting Arduino Uno board to IMAC

I have not been able to connect my Arduino Uno board to my IMAC. I have OS 10.8.5.
Previously, several weeks ago, I had connected it without a problem but when I try it now the computer does not recognize the board. I recently upgraded my OS. I can't remember if my last attempt to connect the board (and it worked) was before the OS upgrade.

If your Arduino "UNO" is a third-party clone it may need a new driver. Check the chip near the USB connector. If it's not an Atmel chip you will need a driver, typically FTDI or CH340.

It's not a 3rd party board.

It may still be worth checking the serial chip as john suggested. Which one is it?

Which one is your IMAC setup to talk to?

When you run System Information does the board show up under Hardware/USB? If not, try a different USB cable and/or different USB port. Have you tried re-starting your Mac? Doe the power LED light up when the Arduino is connected via USB?