Depending on the redundancy level of a disk group and how you define failure groups, the failure of one or more disks could result in either of the following:
The disks are first taken offline and then automatically dropped. In this case, the disk group remains mounted and serviceable. In addition, because of mirroring, all of the disk group data remains accessible. After the disk drop operation, Oracle ASM performs a rebalance to restore full redundancy for the data on the failed disks.
The entire disk group is automatically dismounted, which means loss of data accessibility.