r/Backup

I accidentally deleted the original BACKUP SHOOT folder from a 1TB NTFS HDD. After deleting it, I created a new folder with the same name, BACKUP SHOOT — the only difference was the capitalization.
I used DMDE to scan the HDD, and it detected the old deleted BACKUP SHOOT folder, including the original folder structure and camera filenames such as C0001.MP4, C0002.MP4, etc.
However, the recovered MP4 files won’t play, even though their file sizes appear to be complete.

I make videos for professional work and passion projects. I made the move to a 4 Bay Nas Ugreen system with 26TB sea gate recertified drives (so 102 total TB).
My first question is how do I test these drives to make sure there are no issues with them being recertified before my return window closes to get a replacement?
For what I do, should I do raid 6 vs raid 5. I hate to waste the extra space and is it really that risky only having one drive for backup instead of two? Do peoples hard drives actually breakdown well backing up another one? I genuinely don’t know.

Im looking for an affordable hard drive to store my textbooks and instructor videos on. Classes are expensive and textbooks even more so. I dont think its right that I lose access to that info at the end of the semester, so im looking to store it and look back on it when needed.

Thanks in advance

I'm wondering what is the safest way to backup photos and videos. I have years worth of photos just laying in google photos, but after reading up on peoples experiences with google photos, it's made me anxious. I do have a sandisk (I forget the GB) usb stick, although idk if it's good or reliable. I dont know much about storage in general so thanks!