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If you use ALTER DISKGROUP … ADD DISK and specify a wildcard for the discovery string, what happens to
disks that are already a part of the same or another disk group?

The command fails unless you specify the FORCE option.

The command fails unless you specify the REUSE option.

The command must be reissued with a more specific discovery string.

The other disks, already part of the disk group, are ignored.

Refer to an Oracle Example for adding disk to diskgroup (link)
How A Disk is Discovered
While an ASM instance is initialized, ASM discovers and examines the contents of all of the disks that are in the
paths that you designated with values in the ASM_DISKSTRING initialization parameter. Disk discovery also
occurs when you:
Query the V$ASM_DISKGROUP and V$ASM_DISK views
Disk Discovery Rules(link)
The rules for discovering ASM disks are as follows:
ASM can discover up to 10,000 disks. That is, if more than 10,000 disks match the ASM_DISKSTRING
initialization parameter, then ASM discovers only the first 10,000.
ASM only discovers disks that contain a partition table.
Note:ASM does not discover a disk that contains an operating system partition table, even if the disk is in an ASM
disk string search path and ASM has read and write permission for the disk.
When adding a disk, the FORCE option must be used if ASM recognizes that the disk was managed by Oracle.
Such a disk appears in the V$ASM_DISK view with a status of FOREIGN. In this case, you can only add the
disk to a disk group by using the FORCE keyword.
In addition, ASM identifies the following configuration errors during discovery:
Multiple paths to the same disk
In this case, if the disk is part of a disk group, then disk group mount fails. If the disk is being added to a disk
group with the ADD DISK or CREATE DISKGROUP command, then the command fails. To correct the error,
adjust the ASM_DISKSTRING value so that ASM will not discover multiple paths to the same disk. Or if you are
using multipathing software, then ensure that you include only the pseudo-device name in the
ASM_DISKSTRING value. See “ASM and Multipathing”.
Multiple ASM disks with the same disk header
This can be caused by having copied one disk onto another. In this case, the disk group mount operation fails.

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