Most people are puzzled as to how much hard disk memory they need. As they say, bigger is better. However, do you really want to store everything in one device? All your files, videos, photos, movies, TV series, etc., in one storage disk? I suggest that if you want to have a huge amount of storage capacity, better to keep them separated. Example, dedicate one HDD for your files and photos, and another for movies and TV series. That way, the HDD holding your important documents will have less risk of being corrupted or damaged.
If your looking for large amounts of storage and you don't want to worry about losing data, why not go with a NAS (Network Attached Storage)? This would allow you to have both.
Yes, that is one option. However, this will suffer the portability of your files. Unless, NAS can be connected to an SFTP or SVN.
I usually have one big HDD, I'm currently using a 1 TB and just dumping all my files in there. I admit that it's a bit annoying when I need to reformat and I have to move all of these files around. then again there are programs available now for making these things not as annoying. Like Onedrive where you can place files on a private storage on the internet.
I would say more is better. I normally run off of (2) different hard drives using one as primary and one as backup. But speed is also a big deal to, so I think it really depends on what you plan on doing.
I personally just use the one big HDD for each computer, with a portable that works as my backup. It's 1TB so can handle anything I throw at it, and it's very durable to boot-no pun intended-so I don't have to worry too much about it getting damaged. I do need to buy a new cable for it though as it got mildly damaged a while back.