If a disk fails and no hot spare disk that exactly matches the failed disk is available, Data ONTAP uses the best available spare. Understanding how Data ONTAP chooses an appropriate spare when there is no matching spare enables you to optimize your spare allocation for your environment.
The replacement disk is downsized to match the size of the disk it is replacing; the extra capacity is not available.
Using drives with different speeds within the same aggregate is not optimal. Replacing a disk with a slower disk can cause performance degradation, and replacing a disk with a faster disk is not cost-effective.
Using drives from the wrong pool is not optimal because you no longer have fault isolation for your SyncMirror configuration.
If no spare exists with an equivalent disk type or checksum type, the RAID group that contains the failed disk goes into degraded mode; Data ONTAP does not combine effective disk types or checksum types within a RAID group.