ERROR: sdb2/backup.r18 failed verification -- update retained.
This happened when I was rsyncing a dying hard drive, a lot of people seem to report this as an error with rsync itself but I don't think rsync is the issue at all.
I know the source hard drive was bad, it was having all kinds of timeout, seek, hardware ECC recovered and reset errors.
The only time something else could be the problem is if the data is being corrupted in transit, perhaps through an overseas link or a problematic router along the route etc..
rsync, sdb, verification, update, retained, rsyncing, timeout, hardware, ecc, recovered, reset, errors, corrupted, transit, overseas, problematic, router, etc,