Linux

It's sounds like what you're looking for is backup, and RAID is not backup, it's redundancy to maintain uptime (as well as data integrity, and in some cases performance). I'd highly recommend you look into backup options, with best case being a 3-2-1 backup strategy.

To be fair, I'm being a little hypocritical. I've been working on my backup strategy for years and still don’t have any remote backups yet. Personally, I have a JBOD system, with 8 drives ranging from 2TB to 8TB, so my setup might be a bit complicated for your purposes. I’m not worried about uptime, and am focused on data integrity. I’m not using actual RAID because in the case of a catastrophic failure, I don’t want to lose all my data. I use snapRAID to create some redundancy, and I pool my data drives using mergerfs.

If you are still interested in RAID, I would recommend staying away from hardware RAID as I’ve commented in other places of this post. It has it’s place in data centres but really doesn’t make sense for consumers anymore. There is a lot of good advice in the rest of the comments about RAID, so I’ll summarize my thoughts. If you only plan on having 2 drives, RAID 1 is a good option, though it’s generally used for it’s write performance and that’s probably not necessary on a media server. My current server is running on decade old, lower-end, consumer hardware, and even in that extreme case, media sometimes takes a second to start 1080p content remotely. If you want to add drives and are willing to expand in redundant pairs, you you can either add another RAID layer (RAID 1+0), or pool the partitions together. If you want to be able to expand by single drives/have more than 50% of your potential storage be realized, you could look at RAID 5/6 or ZFS/btrfs. Note that for RAID 5/6, drives need to be equal size.