Every hard drive must have a primary partition. Technically you can have nothing but logical partitions but things can get weird.
If you just want to use the old drive for storage I recommend doing a long format and simply letting it have it’s own drive letter. It’s much easier and less stressful than any alternative.
If you really wanted you could format the old drive, convert it to a dynamic volume, and then have the entire drive show up as a folder on your new main drive but I don’t recommend doing that unless you absolutely have to.