ORA-01150: cannot prevent writes - file has online backup set

You will see the following error when you try to drop a tablespace which is having one or more datafiles in backup mode.

ORA-01150: cannot prevent writes - file 6 has online backup set
ORA-01110: data file 6: '+DATA2/ora11g/datafile/newtb.266.772045007'

Solution

Change the backup mode

ALTER TABLESPACE NEWTB END BACKUP;