Key Features Online volume expansion Dedicated local backup solution Hot-swappable drive tray design Synchronised system operation with Disk Station Synology DX517 offers simple storage capacity expansion solution for your Disk Station through its intuitive plug-n-use design. DX517 allows you to instantly scale up to 5 additional hard drives, so that you can expand RAID volume online or deploy the drives as a dedicated local backup destination for your Diskstation.