The caddy you noted is a Generic 2nd Hard Drive Hdd Ssd Caddy for Apple Macbook 13 Inch 2006 2007 2008 Macbook Pro 15 Inch Early 2006(A1150) so that's a slick way for some to add storage if they don't use the ODD.
Is it through a usb? A specific chord? SATA Hdd hardrive enclosure: http://www.amazon.com/gp/product/B00KDZA868ref_=cm_cd_al_qh_dp_t

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