A couple of final thoughts now that we know the situation:
1. Make sure whatever NAS "device" you settle on for Lynn's side formats the HDD with a fs that your computer can understand. If you only have Windows, then make sure the NAS uses NTFS. If you have Linux, then the world's your oyster, and I would opt for a Linux fs, but NTFS will be ok with ntfs3g.
2. Whatever the Seagate MTBF says, take it with a grain of salt. Look carefully at the box/instructions/specs to see if it says "not for always-on use". If you have some funky special-grade device, I'd be curious to know the model #.
3. John and Adam are right: back the thing up. Buy a 2nd 8TB USB drive, the cheapest one you can find (often there are sales that are very attractive on external drives). Just do a byte-level copy of the whole thing, or a file-level backup, every few months (i.e. no RAID). The NAS device you settle on might have an option to do this for you. I will virtually guarantee you your drive will die before you get a hankering to replace it, even if you haven't personally experienced such a thing.
The very worst thing to lose is the drive full of irreplaceable family photos.
Also, John's Pi solution might be ideal if you don't want to wing it, and even better if someone makes some nice NAS OS images that you can just plop onto it?? For your use case, performance limitations won't be a big deal.