Card reader gets lost

I have a problem with a card reader installed in a computer. It is pluged into on of the available usb pinouts on the Mobo. The card reader works to view whatever is on the memory card but when you pull the card out the card reader drives all disapear and the computer needs to be rebooted in order for it to be found again. I have changed the card reader and the problem still exsits. Any help would be greatly appreciated. Thanks