Hey there, @Tylor Schuessele!
Unfortunately, I'm afraid that this setup is not really what you are looking for. If you want to backup your old photos and videos, I'd advise you have multiple duplicates of these media files stored in different locations on-site as well as off-site. RAID 1 is not a backup solution, if you don't have a duplicate of the files stored on the RAID 1 somewhere off-site (externally) as well!
Another reason why this won't work is the fact that the 8 TB HDD will be limited to the capacity and capabilities of the 2 TB HDD. This means that you will be able to use only 2 TB out of those 8 TB which definitely not an optimal storage setup. You cannot RAID 1 two different drives without consequences, that's why it's highly recommended to use identical RAID/NAS-specific drives that are suited for such array environment.
My recommendation is to consider using one of the HDDs in your system and have the old videos and photos stored there and keep a copy of them onto the other HDD externally (you can put it in an enclosure or docking station). If you do have other external storage devices, you can use them as well. The more copies you have, the safer you will feel about your data. Having the data stored only onto the RAID 1 array and solely inside the PC means that it's still in jeopardy because you never know what might happen to the PC.
Hope this helps you. Keep me posted if you have any additional questions.