Ubuntu does not need a separate hard drive to be installed on, however, it needs its own partition on a hard drive. When you choose the option "Install alongside Windows" during the Ubuntu setup, your Windows partition will be resized automatically and Ubuntu will place its own partition in the created free space.
Of course, if you wish to install Ubuntu on a second hard drive, this is no problem at all. Just select the second hard drive as the destination drive during the Ubuntu setup.
As a result, you can decide which operating system to boot from the GRUB boot menu, which appears after the BIOS POST.