Currently trying to install Ubuntu on my Windows 10 desktop to be dual-bootable. I'm following along this installation guide. I used the guide from Ubuntu's site to create my bootable USB, I did that successfully.

Current situation: I've booted from my UEFI USB, started the install process, and now I'm at the part in the guide: "Figure 3: Advanced partitioning tool of Ubuntu". My free space shows 0MB, so why is it not showing either of my hard drives as having free space for this? Or did the guide skip over something crucial where I have to tell it where to look for this space?

Picture of my partition page.

General install question: The guide says "it will not create a separate partition for your files and folders" if I select the first option, which is to install it alongside the Windows Boot Manager. What is the effective difference here? Would they really make it the number 1, easy choice if I couldn't save things? Or am I misunderstanding that?