Hello, & Welcome to Tom's Hardware!
On your MB, make sure your main Win-7 HDD is on one of the earliest Sata ports, like SATA_0, and this larger driver is on SATA_1 or SATA_2.
Then in the BIOS, check to see that the Win-7 HDD is Disk0, and the larger drive is Disk1 or Disk2. Also since both have OS's on them, make sure the Win-7 drive is 1st in the boot order. (first choice of the hard drives listed).
That should let it boot up to Win-7 without having the boot files on the larger drive interfere.