RAID is a technique for making multiple hard drives in an enclosure behave like they're a single drive. Depending on the type of RAID, you can speed up data access or create redundancy for your data. Here's a full list of all the RAID types, but the one you want for preservation is RAID 1. This duplicates your data onto two drives, so if one goes down no data is lost and you can replaces the downed drive with a fresh one seamlessly.
You'll need a hard drive enclosure that is capable as being configured as a RAID. The one I use is linked. Beyond that, you'll want to consider the 3-2-1 backup strategy. The "1" represents one off-site backup of your data. My solution for that is Backblaze.
I also find the Transmission app is rather useful for preserving media as well.