Fair use does allow you to back up your stuff. That's why you can rip CDs in the US. Nbd. They tried to make it stop, but it didn't. And I buy CDs specifically to rip the lossless audio off of them. You can also rip DVDs legally.

But Blu-Ray gets a little muddy. It's not the content that's the problem, but the DRM. You're allowed to back up the content, but it's technically illegal to defeat the DRM in order to access said content.

Hasn't stopped me. I don't make my MKVs available on the Internet. I acquire copies of my movies legitimately. What I do with the discs when I get home is honestly nobody's business.