My intention is to run a web and file server with Raspberry Pi (rpi). The micro SD-card is unfit by its capacity and lifetime (write cycles). To avoid that the primary SD-card will wear out too fast, we have to extend the storage. Raspberry Pi offers the capability to connect to further external storage over USB. I assume to keep costs low, they offer only USB 2.0 instead of USB 3.0. Since I have a two old external USB 2.5 HDD and an USB 2.0 Hub it fits perfectly the purpose. The USB hub has a separate power supply and powers the external hdd. The hdd are connected with the rpi over the hub. As side note, you could also use a USB 3.0 external HDD, but it will operate with USB 2.0 speed.