A few years back we had a major power outage within our building that shutdown the data center. Upon restore of the power and services we discovered an issue within ASM when using EMC Clarion. Apparently when you mirror externally the FAT that is contained on the ASM disk is not included nor is there a second copy anywhere else. If you lose the FAT which is the first part of the ASM disk and can be seen with DD command, your only resolution is a restore of the entire diskgroup and recover of the database. Ouch. It is resolved in 10.2.0.5. If you lose the FAT there is a second copy at the end of the ASM disk that can be copied to the start of the disk.
If you are still on 10.2.0.4 at least upgrade to 10.2.0.5 the terminal release, preferably upgrade to 18.104.22.168 and begin preparing for 12c. Having worked in environments where upgrades don’t necessarily happen quickly due to regression I understand that some times you have to do the minimal in order to avoid a major outage.