Data ONTAP temporarily stops I/O activity to a disk and takes a disk offline when Data ONTAP is updating disk firmware in background mode or when disks become non-responsive. While the disk is offline, Data ONTAP performs a quick check on it to reduce the likelihood of forced disk failures.
While the disk is offline, Data ONTAP reads from other disks within the RAID group while writes are logged. When the offline disk is ready to come back online, Data ONTAP re-synchronizes the RAID group and brings the disk online. This process generally takes a few minutes and incurs a negligible performance impact.
You view the status of disks by using the aggr status -r or aggr status -s commands.