If you don't want to mess with the back up HDD then don't.
You will need at the least two partitions on your main HDD(the Windows recovery is on the other, yes?), Your W7 partition and Mint. You may also want a /home partition as this makes it easier to upgrade to a new version of Mint when the time comes. Also there is the matter of a swap partition. If you use hibernation you will need it. It is generally advised to have one regardless as there are a few instances where Linux expects there to be a swap. Some people don't use one and have no issues though. If you do create a swap 1.5*RAM is sufficient, though some say 2*RAM.
You can do the partitions yourself or the installer can do it for you. I'm not sure if the installer can make a separate /home though.
I would go with partitions like this
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Formerly known as Luckydog.
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