Any recommendations for a 20TB hard drive?

IMO just get whatever the cheapest one is of those big manufacturers. You should be running some sort of redundancy for your disks anyway, and disk failures are always a gamble no matter what you do to pre-emptively stop them. Personally I buy cheap refurbished drives and throw them into my RAID with the foregone conclusion that I might need to replace them sooner than a new drive, but I'm also saving so much money by buying refurbished that replacement cost will be cheap. Check ebay or ServerPartDeals if you subscribe to this line of thinking.

Edit: This would be sort of similar to "cattle not pets", where you strategize for failure instead of trying to prevent it from failing.