I have some rare media that I know would be extremely difficult to replace, so I back that up, but the general stuff is less important.

However, with rights holders constantly trying to move away from the idea of permanent physical ownership, some media will become harder and harder to find in their best or purest forms, disks will go out of print and the used market will start to slowly die as media ages and rots.

I mirror it 1:1
If you can’t afford to mirror it, backup the files that were and are currently the most difficult to get atm.

I backup my music, photos, docker settings and that’s about it. Daily backups to one external HDD, but recently setup a second backup that’s runs weekly juuuuust in case. The music is only because it’s taken me a long time to build upy library, and that would be painful to lose. TV, movies, meh.

I just want to add, you may not need to backup what you think you can redownload easily.

There are cases of series or movies being really difficult to find or it takes you a long time to find a working torrent. For those it definitely should make sense to back them up, but its a bit subjective feeling which ones those might be.

Another thing in my case, I got movies and seriea for my child on the server. He expects that is accessible, always, and is too young to understand that data can get lost. So I am also backing that up, it could be easily replaced but it would take me time and this would result in my child being unhappy. Not worth it!

You think you can get the media again if need be.

Depending on how large your collection is, would you remember every item in it? How much effort did you put into organizing it?

IME it’s far more of an inconvenience and expense rebuilding data from scratch than properly backing it up. And the peace of mind from a robust, tried and true DR process is golden.