If I read your post correctly, you built a new computer that includes an SSD on which you have Installed Win 8, plus a HDD you intend to use for data, etc. I suspect strongly that the trouble is that you have not prepared that new empty HDD for use by Windows. That is a process necessary for an empty new drive.
You already know how to look at it in Disk Management, where you can see the unit as all Unallocated Space. Make SURE you are operating only on the HDD unit that says it is all Unallocated Space, not any other unit. RIGHT-click on that Unallocated Space block and choose the option to Create a New Simple Volume. You probably want this volume to include all the space on that unit. Other optional settings like File System and Formatting are likely set OK for you, so just let it do the job. It will take a long time - many hours - so be patient. When it is finished, back out of disk Management and reboot. The HDD should show up now in My Computer.