So I built a new system and got a SSD drive as my main HDD. Do I have to format my old SATA drive to use it as a slave drive? I have a lot of data on it that I really dont want to lose.
Just make sure your SSD is the first on the boot order.
I never did, haven't had problems with it
If the radiance of a thousand suns were to burst at once into the sky, that would be like the splendor of the Mighty One... now I am become Death, the Destroyer of Worlds.
You can also just delete the hidden/system boot files in the root folder on your old HDD, if you don't want to boot from it. All other files can remain.
Otherwise, as the others said, just be sure to set your SSD to boot first in BIOS. Installing your OS with your old drive disconnected is a good idea too.