Going crazy trying to Clone Main drive to new upgrade drive

Okay I'm at my wit's end here.

 

I have a 400 GB mechanical HD with windows 7 on it.

 

I purchased a Seagate Momentus XT 750 GB as an upgrade.

 

I am trying to clone the old drive to the new one however when I follow the instructions and ensure that the new drive is unallocated space and doesn't have a drive letter set it will not allow me to click "COPY MBR" - the option is greyed out.

 

When I select a drive with an assigned letter I can copy the MBR but I know that this will cause issues when trying to set as the primary later on if I assign the new drive a letter!

 

Please help!