My guess is you want to new hard drive all under one partition because that's what you're familiar with. But, partitioning a drive can be advantageous and many people prefer to have multiple partitions. A general scenario is that the C partition would be where Windows is installed and other programs. But, data files, photos, music, etc., could be stored on one or more other partitions.
That 20GB partition that was created on your new drive would be fine for Windows and programs; 20GB would be adequate for Windows and programs for most users. If more space is needed, programs can be installed on another partition.
My point: why not leave the 20GB partition as is, and create one or more partitions on the rest of the drive space?