You replace disks that are currently being used in an aggregate to swap out mismatched disks from a RAID group. Keeping your RAID groups homogeneous helps to optimize storage system performance.
You should already have an appropriate hot spare disk of the correct type, size, speed, and checksum type installed in your storage system. This spare disk must be assigned to the same system and pool as the disk it will replace. If the disk to be replaced is partitioned or in a storage pool, the spare disk (or its partitions, if applicable) can be owned by either node in the HA pair.
For multi-disk carrier disks, you should have at least two hot spare disks available, to enable Data ONTAP to provide an optimal disk layout.
If you replace a smaller disk with a larger disk, the capacity of the larger disk is downsized to match that of the smaller disk; the usable capacity of the aggregate is not increased.
If you need to stop the disk replace operation, you can use the -action stop parameter.