TL;DR - Do the faster and the original preclear scripts continue the post-read all the way to the end even if it has already read non-zero data?
Longer story
I just finished (finally, successfully) preclearing a 6TB WD Red with the faster preclear. All attempts were using the plugin but I don't think I would have had a different result if I had run the scripts from the command line.
The 1st attempt, I did 1 complete cycle, and after it was finished, it said the disk passed, and the smart data was good, but the post-read had detected non-zero data.
The 2nd attempt, I skipped the pre-read and just did the clear and post-read. Same result.
The 3rd attempt, I just did the post-read, but with the original script. Same result.
Then, I decided to change my connections a little since I had been using an eSATA enclosure on an expansion card, and my intent was to put the drive in as parity using a motherboard port. I didn't expect that to make any difference since I had precleared several drives like that in the past, just nothing larger than 4TB.
And I don't think the connections did matter, because I discovered what I think may have been the actual problem. Before booting up after the changes, I did a memtest and discovered that 1 of my 2x8GB memory sticks was bad. I tested each of them separately in both slots and it was definitely that 1 stick.
After removing that memory and just running with only 8GB RAM, I did the preclear again.
This 4th attempt skipped the pre-read and just did the clear and post-read with the faster script. Success!
But, see my question at the top.