toshiba 3tb

I'm new to building PCs, but I built one this week (here's the part list in case that is useful: https://uk.pcpartpicker.com/list/yq7bt8). I installed Windows 10 65Bit onto the SSD, and now I can't access the 3TB Toshiba p300 HDD while the computer is running. It's really confusing me. I know...