cascade on update using triggers

create or replace trigger aartri
after update or delete
on disks_337517
for each row
begin
update ssimple set disk_id=:new.disk_id where disk_id=:old.disk_id;
end;
/

if i execute my trigger gets created....
i have a value for disk_id 4 in both the tables...
when i tried to update..
update disks_337517 set disk_id=9 where disk_id=4;
am getting error,
integrity constraint (SCOTT.DID) violated - child record found