New OS on 2nd internal SATA drive.

The instructions I'm providing are dependant on you having a full Windows 7 OS, NOT an upgrade.

First you will need to get rid of the dual boot option. Start up in Windows 7.
Click on start, in the search box type msconfig a popup will display a list of files the top one will be "msconfig" click on it and a box will open up, choose the boot tab, highlight the Windows 7 choice and click on delete then click on apply.
Restart in your other OS and quick format the Windows 7 drive.
Next you will need a USB thumb drive of at least 4 GB capacity to use for Windows 7 instalation files.

Your computer must be capable of booting from a USB drive!

How to install W7 from a USB pen drive.
http://www.intowindows.com/how-to-install-windows-7vist...
When you have your bootable USB drive with W7 ready, your next step is to shut down the PC, open the case and disable the drive containing the OS by unplugging the data cable.
insert the usb stick, start the PC and enter the bios screen, choose the usb drive as first boot save settings, when it restarts and boots the usb drive start the W7 instalation.
If all goes well it should install in about 15-20 minutes assuming you have a reasonably fitted out computer, if it's a bit old and slow it could take longer.
Once you have completed instalation and installing your programs shut down and reconnect the other drive.
You will be able to select the OS by enering the bios and choosing the first boot device that contains the OS you want use, they will be totally independent.

IF you simply want to upgrade your current OS and not have an option to dual boot at all, use the make bootable USB procedure and run the upgrade on your current OS drive by booting the USB drive and coose custom option to do a clean install, you will not be able to install on the 2nd hard and not have a dual boot as your current system is a requirement of the upgrade.
What you can do is use the hard drive makers tools to migrate the system to the new drive, WD provide a WD version of Aconis True Image for this purpose, other makers also provide tools to so.