Right-click on Computer or This PC and select Manage, on the left column click on the Disk Management.
On the right side right-click over the partition that you want to mark as active (below the blue section) and choose Mark Partition as Active.
Format the partition
If it does not work or you are not able to complete the process then you could perform the same task using Diskpart in the Command Prompt.
Open a command prompt as administrator.
Type diskpart hit Enter
Type list disk hit Enter. You will see a list of the disk on you PC.
Type select disk # (replace # with the number on the disk you want to format) hit Enter.
WARNING: The next step will delete all data on the selected...