I have olympus e300 evolt and the read out says card cover open when it is closed any ideas?

Inside the card cover, there are two 'hooks' and one small protruding pin. This pin will push a (very small) brownish contact pin that can be found near the card eject button. If either of the pins is damaged, the contact pin will not be pushed in far enough and the camera will not get the 'card cover closed' signal, which results in a 'card cover open' warning message. You might be able to fix this problem by inserting a (very small/thin) piece of cork in the contact pin hole, so that the contact pin is again pushed in far enough to close the contact.
Just to make sure this is the problem, try pushing the small contact pin in with a pen, pencil or small screwdriver. If the warning message disappears, you know the contact pin is still working and is probably not pushed in far enough.

My Cannon has to be well clipped in ... scares me doing it but they can be a little tight and of course must be the right way around ... most are lug prevented from misplacement ... then of course I cannot get it out ! later on, just a guess . Mike could be corroded, grubby electrical connect as it is shut ?