It's recommended to use different hard disk for Ubuntu if you are planning to dual boot it with other operating system. In this step, you have an option to choose which hard disk you are planning to put Ubuntu in. You'll see a FAT or NTFS formatted partition if you already have Windows in your computer. If you have created a free space in your hard disk reserved for Ubuntu, it will appear here. If not, choose hard disk to create a partition first.

We have chosen a whole hard disk as an example in this tutorial. Normally, people does not dual boot Ubuntu server. So, we are going to choose a hard disk and create a partition for Ubuntu server in the next step.