In article , Peter Jason
wrote:

The old faulty 4TB drive I quick-formatted after
which this also chkdsked OK & was recognized by
Macrium too.

zeroing all sectors will remap any bad blocks.

So it was some software problem on the old HDD,
and I wonder what this was & how I account for
this in the future. Was it some partition problem
& is there some diagnostic for this?

check smart data, although it may not be conclusive.