SD Card problems

I have the G Tablet and only had a 512mb mini card to start. It worked fine. I ordered two new Kingston 8G cards to get extra capacity but when I replace the 512 with either new one I get a message that says the card is damaged and I need to format it. I tell it to do that but nothing happens. Any ideas with what might be going on? Also, both cards work on my PC when they are in their adapters.

 

Many thanks. That "seems" to have worked - no error messages. I still don't have the navigation down pat yet as too how to get to the disks/folders, etc so I am not totally sure. ES File Explorer buttons, and what they do, are not all clear to me.