The rules for discovering Oracle ASM disks are as follows:
Oracle ASM can discover up to 10,000 disks. That is, if more than 10,000 disks match the ASM_DISKSTRING initialization parameter, then Oracle ASM discovers only the first 10,000.
Oracle ASM only discovers disk partitions. Oracle ASM does not discover partitions that include the partition table.
From the perspective of the installation, candidate disks are those that have the CANDIDATE, PROVISIONED, or FORMER header status. These disks with a CANDIDATE, PROVISIONED, or FORMER status can be added to Oracle ASM disk groups without using the FORCE flag.
When adding a disk, the FORCE option must be used if Oracle 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.
MEMBER disks can usually be added to a disk group by specifying the FORCE flag, if the disks are not part of a currently mounted disk group.
In addition, Oracle 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 Oracle ASM does 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 "Oracle ASM and Multipathing".
Multiple Oracle 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.