|You obviously didn't use the 2nd HDD for data like you were supposed to - lol!|
Right click on My Documents, click Properties, then click the General tab. What's listed for "size on disk"?
Do you regularly empty the Recycle Bin? Until you empty it, those files are still occupying space on the drive.
Do you use a cleaning program such as CCleaner-Slim? If not, I suggest you download & install it. You can find it at the following page, last one on the list:
After you install it, click on the desktop icon. When it opens, look at the left column. Uncheck the boxes next to Cookies & Empty Recycle Bin (you can handle both of these manually) then click "Run CCleaner". That should clear out a lot of garbage.