> - Read manufacturers' claims and storage conditions.
> - Make 2 (minimum) copies.
> - Test read at no longer than half lifetime
> - If any degradation, make new copies of the copies
> - If no degradation, make new copies anyway :-)
> - Maybe make a few test disks which are read say yearly to  see if there is
> a noticeable trend.

Another important point:

- select new storage media before your backup system becomes unsupported history.

After all, the discussion was about 50-100 years of data retention.  Think
about restoring a hypothetical <20 year old 8" disk backup today?!   And of
what use is the data today?