Since 5.5 FLUSH TABLES requires unlocked tables so the old LOCK TABLES/FLUSH TABLES
    model fails. Use atomic FLUSH TABLES WITH READ LOCK instead.

per-file comments:
  scripts/mysqlhotcopy.sh
Bug#42465      mysqlhotcopy fails with "Table '<y>' was locked with a READ lock" error message.
     Use FLUSH TABLES WITH READ LOCK instead of LOCK TABLES / FLUSH TABLES pair.
     Keep the old scheme for older servers.