Your SSD and HDD cannot have the same drive letter;
If you do not see an extra hard drive (likely named "D:") then your HDD may not be connected properly.
you have an overview of your drives in diskmgmt.msc
Perhaps your HDD is connected but not initialised, in which case it can also be done in diskmgmt.msc
Once your HDD is installed and initialised and formatted properly, it will have its own drive letter which you will be able to select from any game installer.