A normal hard drive on a PC consists of several moving parts.  Not only do these consume more power to run, but they are very sensitive to motion as all the parts must work in exact synchronisation in order for the data to be read, meaning a hard drive is not ideally suited for use on a pitching boat in heavy seas.
A solid-state drive has no moving parts at all - data is stored in Flash memory.  As there are no moving parts, a solid-state drive consumes less power and performs well under all sea conditions.