My wired USB keyboard is not recognised on my iMac
Over the last few months I am seeing my iMac freeze up and the only way out is to power down and power up. When I do this the wired USB keyboard is not recognised by the iMac. The only way I have found to get it working is to use the PRAM reset. That does not always work first time and it seems I need to disconnect the keyboard and run the PRAM reset and then connect the keyboard again. The easiest test is just to toggle the Caps lock key. It does not work when the keybaord is not recognised. I am not running much else on the USB bus. I have an external backup hard disk running through Time Machine and a printer. I am sometimes connecting an Arduino to the USB. However this last crash happened when the Arduino was disconnected.
Any ideas what is causing this and why it knocks out the wired keyboard.
For those who have not come across the PRAM reset then it is detailed here. http://support.apple.com/kb/ph11243
Fortunately the Bluetooth keyboard works otherwise the PRAM reset would be inaccessible.
I am running OS x 10.9.2
iMac, OS X Mavericks (10.9.2)