V5.5.36 will only use default data directory

I installed v5.5.36 on a new Mint16 installation and it started ok. I then modified my.cnf to point to an existing data directory. I made sure that all folders and files were owned by mysql user and the mysql group had permissions to read/write. I also deleted existing files ib_logfile0/1, ibdata1 and aria_log_control as I know these must be recreated. When restarting MariaDb I get: Can't create/write to file '/home/michael/mysql/aria_log_control' (Errcode: 13). Files ib_logfile0 & 1 and ibdata1 were created ok so it is not a permissions problems. Why does only aria_log_control fail to create. If I copy all this data to default /var/lib/mysql and reset my.cnf then they work fine