How do I reformat my hard drive?

Most of the time you wont have a problem by just by adding it in after you install your operating system on your main drive. That said, one time I did have a problem with a nearly identical situation that you are trying. I had a old 1 TB drive with the original OS on it. I then removed that drive and installed a clean OS on a SSD then I connected the old 1 TB drive and the computer failed. When I took out the 1 TB the computer booted normally. I am pretty sure this was peculiar to windows 7 since I have been doing things that way for years with older OS's. To be on the safe side, I would connect both drives then do a format both drives using the Windows installation disk. Boot the computer to the installation disk with both drives installed. Using the advanced features of the installation disk, do a >delete partition>create partition>full format in that order. Do this for each physical disk. Then you can proceed with an install.