after upgrading to mountain lion my usb drive gets disconnected

I have a late 2009 iMac that just installed Mountain Lion.

Every single USB external drive that is connected, shortly after a while gets unmounted.

I tried with disabling sleep mode for HD etc and no success.

Then I tried USB card reader and same exactly think. Memory card gets mounted and after about a minute gets unmounted.

What could be a problem or better yet, sollution ?

Any help is appreciated.