Check that they’re actually 8tb. Usually when drives that cheap are opened up, it’s USB sticks that have custom firmware to pretend to have 8tb, and you’ll really only have something like 8gb. It’ll overwrite the data as you keep writing to it, so you may not notice it until a while from now. Even if they feel heavy. Some of them have weights added to throw you off. Take a look at https://www.reddit.com/r/pcmasterrace/comments/9wimlo/when_you_buy_a_cheap_external_ssd_from_china/

(Sorry for the reddit link, but that’s where the images are)