Oracle Solution for Error ora-15029

ORA-15029: disk 'string' is already mounted by this instance

What triggered the Error:

An attempt was made to add to a diskgroup a disk that was already mounted by the current instance as part of some (possibly other) diskgroup.

What should we do to fix it:

Specify a different disk in the command. Note that not even the FORCE option can be used to correct the situation until the diskgroup containing the disk becomes dismounted by this instance.