ORA-01668: standby database requires DROP option for offline of data file
Cause: Attempting to take a datafile offline in a standby database without specifying the DROP option. Files that are offline in a standby database are not recovered, and are likely to be unusable if the standby is activated. Note that specifying DROP does not prevent bringing the file online later.
Action: Specify the DROP option or leave the file online.
Error Type: ORA