"You may access Content for your information and personal use solely as intended through the provided functionality of the Service and as permitted under these Terms of Service. You shall not download any Content unless you see a “download” or similar link displayed by YouTube on the Service for that Content."
I have a NUC server at my parents house (they have a fast connection) where I connect remotely and make all my downloads. When I visit them I bring an external HD with me.
I think something could be said about how an agreement of that manner is not legally binding so you can ignore it, but IANAL. Just a faint memory something from the legality of TOS and maybe I'm not remembering it right. I place my confidence at 85% for "not legally enforceable".
It has to be brought to your attention and usually that step is when creating an account.
I'm also not sure if "please do not use the Service" is equivalent to "you may not use the Service". It's a request, not a demand. But I'm not sure if that sort of pedantic attitude would slide in court.
Regardless - Google/YouTube aren't enforcing this so I don't think it matters from a legal standpoint - just a moral one.
>By using or visiting the YouTube website or any YouTube products, software, data feeds, and services provided to you on, from, or through the YouTube website (collectively the "Service") you signify your agreement to (1) these terms and conditions (the "Terms of Service"), (2) Google's Privacy Policy, found at http://www.youtube.com/t/privacy and incorporated herein by reference, and (3) YouTube's Community Guidelines, found at http://www.youtube.com/t/community_guidelines and also incorporated herein by reference. If you do not agree to any of these terms, the Google Privacy Policy, or the Community Guidelines, please do not use the Service.