Network Attached Storage (NAS) is a file storage device where you can safely store, manage and access files from any of your devices. Ideally, it will be a very-low energy consumer as it will always be on.
My NAS stores back-ups of all my digital photographs, files and high quality (FLAC) music files. The music files are accessed by a DLNA HiFi media player. Files and music are instantly available from any device in home of when away from home.
There are many very good NAS devices on the market but most are expensive for what they do and not particularly energy efficient.
I have chosen a Banana Pro single board computer connected to a typical laptop hard drive running Armbian, a Linux Operating System - More. This credit-card-sized computer uses just a couple of watts (compared to hundreds of watts for a normal PC) and is completely driven by home produced renewable energy (with battery backup).