
Synology RAID Calculator makes recommendations based on the total capacity picked.To find out more, please refer to Why does my Synology NAS have a single volume size limitation of 16, 108, or 200 TB? Some models can support larger single volume capacities after upgrading the installed memory capacity. In cases where you are over the limit, you may create multiple volumes or designate a drive as a hot spare. Models listed under "Other models" support the selected number of drives and their capacities but may have limitations on how large you can create a single volume.If you're going to expand an existing volume, please read Expand Volumes or Disk Groups by Adding Hard Disks and Expand Volumes or Disk Groups by Replacing Hard Disks for detailed information. The result of calculation helps you evaluate the available space when you are creating a new volume only.

On the contrary, Unused space under other RAID types indicates the space that can't be utilized and is thus wasted.

Visit here for more information on choosing the correct HDD for your Synology NAS.
