If you installed W7 with the other drive connected and Windows created a dual boot, then you may have systems files on your second drive preventing you from formatting it. I also doubt if Windows will allow you to delete the OS on that drive if there are system files on there that it's using to boot with.
If you havn't got too much installed on it yet, it may be easier and quicker to reinstall W7 with the second drive disconnected. You can then safely delete the old OS on it leaving your data files.
The other option would be to repair the boot files on the ssd after first disconnecting the second drive. This can be a bit fiddly.
If you want to go this route then have a read of this article on repairing the bootloader.http://www.windows7hacker.com/index.php/2010/01/how-to-manually-troubleshoot-and-repair-windows-7-bootloader/