disk not readable

Hi, I installed succesfully OS X El Capitan, but whenever i start/restart the computer I get the message "The disk you inserted was not readable by this computer", with three options: Initialize, Ignore and Eject. If I click on Initialize, the Disk Utility opens and shows that my secondary...

Every time I start or restart my computer I get this message , The disk you inserted was not readable by this computer. i have the option to eject, initialize, or ignore....why is this and what can I do to stop this from occurring?