Is there any specific reason for using a HDD? Limit of disks supported by your server? Cost?
I second @[email protected] for using an SSD for streaming. Compared with the movies, the size of Ubuntu is relative small. Even if you can tolerate the boot up speed, I don't think it worth the extra complexity of another HDD
EDIT: As OP in https://lemmy.ca/comment/372018 mentioned that OP already selected a 8TB HDD, I think it's totally okay to use the HDD for system for simplicity.