qBittorrent official forums

16 MB piece size should only happen with torrents over 100 GB in size. Someone manually chose that value when creating the torrent.
It also means roughly 4 files share a single piece. Perfect combination for the 2 computers to have wildly different percent completes.

Yes, the trackers are probably reporting many of the same peers. If it were 5 trackers reporting 30 peers each, there might be fewer than 50 real peers. It's even possible to be fewer than 30 peers -- as trackers sometimes report dead peers or peers that have multiple ip addresses (IPv4 and IPv6 for instance).

Not to mention peers that send a different 'randomised ID' each time they announce to a tracker.

[quote="Switeck"]
16 MB piece size should only happen with torrents over 100 GB in size.
[/quote]
My bad, it's 500GB, not MB

[quote="ciaobaby"]
Not to mention peers that send a different 'randomised ID' each time they announce to a tracker.
[/quote]

OK, that could be related in another way.  Are the peers keeping track of what pieces they have already sent to other peers?  That could explain why it won't resend pieces. 

For the purposes of simple explanations, I've kept the my description brief.  Maybe I need to give more details.
I actually am working on 7 very large torrents, (5x500gb, 1x400gb, 1x700gb) Between them, there are a lot of duplicate files.  I've been copying files retrieved by one torrent into the others's directory, then forcing recheck.  Obviously it's less than fully efficient, that way, but, I'm trusting the recheck function to make sure that anything that can't be verified is kept out of the percentages.  I've been able to complete quite a few of the files that way, or at least get significantly higher percentages.

When I introduced a new peer into the mix, it talked with everything and acted as the middle man.

[quote="dwight"]Are the peers keeping track of what pieces they have already sent to other peers?  That could explain why it won't resend pieces.[/quote]While peers send each other HAVE piece messages, that information is lost/forgotten if the torrent is stopped/restarted or qBT is closed/reopened. There isn't any protection as far as I know for a peer to request the same piece over-and-over again due to hash fails.

What you're doing I've done myself in the past. Painfully slow rechecking all those files, but better than trying to re-download GB's!
Make a backup of qBitTorrent's settings while qBT is shut down in case it crashes and loses your torrent list!

Watch out for different torrents sharing the same files at the same path location! Even if the files are identical, 1 torrent can lock other torrents from writing to it while it tries to write to it. It may even block reads from it at the exact moment data is being written to it. Also, if a torrent is made under Linux or Mac OS, there could be files named "THIS.MP4" and "this.mp4" in the same folder...which Windows will treat as the same file despite the case differences.