mysql Relay log file not found error while changing mysql data directory
From the manual:


  An absolute path remains unchanged; in such a case, the index must be edited manually to enable the new path or paths to be used. Previous to MySQL 5.6.5, manual intervention was required whenever relocating the binary log or relay log files. (Bug #11745230, Bug #12133)


So if your binlog index was written with the full paths, you need to update it manually or using the mysqlbinlogmove utility.