The latest version of macOS, (High Sierra), for example, takes up a little more than 8GB, while the latest version of Windows 10 takes up about 15GB.
First, your operating system is taking up space. The most important thing to know is that the amount of usable storage space a laptop actually has is much less than what is advertised, for two reasons.
This model connects via USB and is compatible with both Mac or PC. LaCie's line of Rugged hard drives are shock- and water-resistant and come in different models with various connections and storage capacities - so get the one that works for you.