ALTER TABLE fh_download
PARTITION BY HASH (id) 
(
  PARTITION partition1 ENGINE = INNODB,
  PARTITION partition2 ENGINE = INNODB
)

A UNIQUE INDEX must include all columns in the table's partitioning function

CREATE TABLE ***.fh_download (
  id int(10) UNSIGNED NOT NULL AUTO_INCREMENT,
  unique_key char(13) DEFAULT NULL,
  PRIMARY KEY (id),
  UNIQUE INDEX IX_unique_key (unique_key)
)
ENGINE = INNODB

CREATE TABLE tbl1 (
  id int(10) UNSIGNED NOT NULL AUTO_INCREMENT,
  hash char(32) NOT NULL,
  PRIMARY KEY (id),
  UNIQUE INDEX IX_file_hash (hash, id)
)
ENGINE = INNODB
CHARACTER SET utf8
COLLATE utf8_general_ci
PARTITION BY KEY (`id`)
(
PARTITION p0 ENGINE = INNODB,
PARTITION p1 ENGINE = INNODB,
PARTITION p2 ENGINE = INNODB,
PARTITION p3 ENGINE = INNODB,
PARTITION p4 ENGINE = INNODB,
PARTITION p5 ENGINE = INNODB,
PARTITION p6 ENGINE = INNODB,
PARTITION p7 ENGINE = INNODB
);

INSERT INTO tbl1 VALUE(NULL, '11111111111111111111111111111111');
INSERT INTO tbl1 VALUE(NULL, '11111111111111111111111111111111');
INSERT INTO tbl1 VALUE(NULL, '11111111111111111111111111111111');