Is this data that you could redownload easily enough if the drive, or raid was lost? A plex server full of movies is generally going to go into this category. If so get multiple 4-5tb drives as these will be the cheapest per gb. You could buy a 5tb one now, and then when you are running low on space add another drive. Eventually it will be worth upgrading the whole system because of power savings and better performance. A raid configuration is going to be more expensive per gb than just having a few drives

Is the data very expensive to replace or one of a kind? Photographs and home movies would fall into this category. First you should have a backup. Crashplan and backblaze are good options for consumers. On a smaller scale cloud based backups like google drive or dropbox work for this. These will protect you not just hardware failure but malware like cryptolocker, accidental deletion, and total computer failure. If you have a large amount of data like professional photography or video production a NAS with RAID 6 is advised. You can lose 2 drives without losing any data, but you lose 2 drives of usable space for parity, and you have to deal with the overhead of setting it up.