If you're looking for an expandable storage solution for backup or for a home media server, Synology's DiskStation is a great option. These DiskStations contain 2 or more hard drives, and connect to your network to share data with multiple computers or devices. This is referred to as NAS, or Network Attached Storage.
The Synology DiskStation DS213j is a 2-Bay NAS server, capable of containing up to 12TB of storage. It can be set up to mirror data between the two drives (RAID 1), which helps to protect data. Or it can be set up to use both drives to double the storage capacity (RAID 0), which is also known as "scary RAID" because all of the data could be lost if one drive fails. The Synology DiskStation supports Windows Backup for Windows users and has native Time Machine support for Mac users.
Synology's DiskStation also makes for a great home media server. The DiskStation's Floating-point unit CPU speeds up thumbnail processing which enhances photo viewing at various resolutions, and it has 512 MB of RAM for running multimedia applications. It also is a DLNA Certified media server, which can stream music, photos, and videos to other DLNA compliant devices like TVs and gaming consoles.
Synology's 2-Bay DS213j is diskless, meaning it does not come with hard drives installed. It accepts any 3.5" or 2.5" SATA III or II HDD, and can also support a 2.5" SATA SSD.