It is important to have multiple (at list two) copy of each online redo logfile. To avoid the disaster of disk failure, it is recommended to place each member of a specific online redo logfile group in different disk.
To view to current online redo logfile group members
SELECT GROUP#,STATUS,MEMBER FROM V$LOGFILES;
Suppose we have disk01 (mount point /u01 ) that contains current online redolog files .To add member in redo logfile group 1,2 and 3 in disk02 ( mount point /u02 )
ALTER DATABASE ADD LOGFILE MEMBER '/u02/redo01.log' TO GROUP 1;
ALTER DATABASE ADD LOGFILE MEMBER '/u02/redo02.log' TO GROUP 2;
ALTER DATABASE ADD LOGFILE MEMBER '/u02/redo03.log' TO GROUP 3;