I have several 8TB Seagate drives, and generally they are
quite reliable. That being said I have WD's as well, just
not so many, and none of those have died as yet.
What are you looking to do with the new drive, carry it
around with or moor it to your computer or even put it
inside
Large or small form factor?? Go for the 3 and a half if at
all possible, I've had several failures of small form factor
4TB drives - maybe because I use them on the karaoke.
Buy new and look for a good long guarantee time, or buy
used and back up