he wants to get rid of windows 7 as he said, for which all you need to do is set hdd with ubuntu installed as a primary boot device in bios (or if there's a priority list for hdds, set the one with ubuntu as 1st in line)
This is true. If the intent is to complete do away with Win 7, then just install Ubuntu to the primary drive. Then add a new filesystem to the second HDD. I would suggest you put /HOME on the secondary drive. That way you can recover your files from it if needed AND you can play around with the primary drive (like trying different distros) if needed.
Still, with 2 HDDs, the OP could leave Win 7 on one and install Ubuntu to the second and be able to support both as needed.
Lots of options here.