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:
✑ Run the ALTER DISKGROUP...ADD DISK and ALTER DISKGROUP...RESIZE
✑ 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
✑ ASM only discovers disks that contain a partition table.
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
✑ 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.