Exadata auto disk management is controlled by the parameter _AUTO_MANAGE_EXADATA_DISKS.

The default value for this parameter is TRUE.

When _AUTO_MANAGE_EXADATA_DISKS is enabled, Exadata automate the following disk operations:

If a griddisk becomes unavailable/available, ASM will OFFLINE/ONLINE it.
If a physicaldisk fails or its status change to predictive failure, for all griddisks built on it ASM will DROP FORCE the failed ones and DROP the ones with predictive failures.
If a flashdisk performance degrades, if there are griddisks built on it, they will be DROPPED FORCE in ASM.
If a physicaldisk is replaced, the celldisk and griddisks will be recreated and the griddisks will be automatically ADDED in ASM, if they were automatically dropped by ASM. If you manually drop the disks, that will not happen.
If a NORMAL, ONLINE griddisk is manually dropped, FORCE option should not be used, otherwise the disk will be automatically added back in ASM.
If a gridisk is inactivated, ASM will automatically OFFLINE it.
If a gridisk is activated, ASM will automatically ONLINED it.

There are some error conditions that may require to temporarily disable _AUTO_MANAGE_EXADATA_DISKS.

Details on MOS 1408865.1 - Exadata Auto Disk Management Add disk failing and ASM Rebalance interrupted with error ORA-15074.

Immediately after taking care of the problem _AUTO_MANAGE_EXADATA_DISKS should be set back to its default value of TRUE.

Full details on Auto disk management feature in Exadata (Doc ID 1484274.1)