Why does my iMac randomly eject thumb and flash drives and then refuse to repair them?

I will be working on the iMac and it will suddenly pop up with a warning saying a disk has been inproperly unmounted even though I haven't touched the thing. I have one through a card reader for my camera and one is a thumbdrive plugged into the back. Now both of these items are un readable with the iMac and it tells me to repair them. I went to disk utility, told it to verify then repair and it is saying it can't repair either of them. This is getting expensive! Please help me.