Remove a disk from an ASM diskgroup

Command

ALTER DISKGROUP DATA DROP DISK DATA_0005 REBALANCE POWER 4;

-- WAIT until the drop completes (the disk can be physically removed once V$ASM_OPERATION is empty):
SELECT group_number, operation, state, power,
       sofar, est_work, est_minutes
FROM   v$asm_operation;

What it does

Removes the disk from the diskgroup; its extents are moved to the remaining disks (rebalance). When complete, the disk's HDR_STATUS becomes FORMER.

When to use it

When replacing a failed disk, reducing capacity or migrating storage.

Caution

Do NOT physically pull the disk BEFORE the rebalance finishes, otherwise data is lost. Make sure the remaining disks have enough free space.