The question title may seem a duplicate, but this case of mine is unique. This happened because of this problem, which I was able to answer/solve. https://superuser.com/questions/1447825/will-re-installing-mojave-through-internet-recovery-delete-data
And now the system files of the
Previous Content, from the previous installed OS cannot be deleted. It seems the MacOS sees these files as legitimate system files, while in fact, they're not.
I'm using Mojave now btw.