Clarify instructions for formatting the hard disk.

[AndrewSouthpaw]

I got stuck for a while trying to figure out how to format the disk. It wasn't intuitive to me to choose [Erase].

I've added some instructions to clarify that step. Maybe it seems super obvious, but to a newbie like me it was an easy step to get stuck on.

I hope the additions are welcome! Happy to make changes as needed.