The Wiz runs Linux, so the card is "mounted" when you insert it into the slot and "unmounted" when you remove it. However if there are any open files on the card when you remove it, the unmount will be refused by the OS to prevent data corruption. It doesn't know the card is physically removable. What this means is that when you then place another card in, the mount will fail as the previous mount was not unmounted.
To best way around this is only ever remove the card once you are on the Wiz menu screen, and also wait a few seconds to ensure all file closes are recognised by the OS.