iMac ejecting USB mass storage devices and DVD's

Hi there, I hope someone can assist me. I am not great with computers at all. I have an iMac which I bought in 2010, I am running OS X mountain lion. It seems that since I did the upgrade to mountain lion my iMac is ejecting any and all disc's and any USB mass storage devices, before I am able to complete copying files to or from the device.
Everything worked fine until I did the upgrade.
Any thoughts on how to correct this will be appreciated.

Do the disc and USB devices show up on the desktop and Finder then just go away? Does the disc actually physically eject from the iMac? Can't imagine what in the OS upgrade would cause this sort of failure..

Hi there, thanks for your response.

Yes the USB devices do show on both the desktop and in Finder and just disappear halfway through my download, and the Discs physically eject. I discovered the disc problem when trying to install Office for Mac so tried again with a music CD to check if maybe there was a fault with the Microsoft disc but the same happened with the music disc. Do you think that maybe I have a virus of sorts on my iMac?

As there aren't any Mac viruses, the answer to your question would be no.

Sounds like the eject command is being invoked involuntarily. Could you be pressing the eject key? Could that key have gotten damaged?

Thanks for your input, I have tried with my USB keyboard and still happening. This keyboard and my printer do not "auto eject" like the storage devices, why would that be?

They don't auto eject because keyboards and printers don't have to be mounted as disk images by OSX. You can't browse a keyboard or a printer.

Sounds like your Mac has a case of the Poltergeists! Have you tried running Disk Utility to make sure everything is in order there? It's sounding like whatever component in OSX that mounts disc images has gone haywire. Might be worth it to just go ahead and call Apple Customer Support...

Try resetting the System Management Controller on your iMac first. Do this:

Unplug your iMac from the AC outlet (shut it down first), and let it set unplugged for about 10 minutes. Plug it back in and reboot. Let us know if that works.

Have you tried running OnyX
Download OnyX for Mac - Maintenance and optimization tool. MacUpdate.com