IBM Storwize V5010 Manual page 170

If the drive fails (as shown in Figure 4-7), the MDisk (from which the failed drive is a member)
remains online and a hot spare is automatically reassigned.
Figure 4-7 Internal drive taken offline
If sufficient spare drives are not available and a drive must be taken offline, the second option
for no redundancy (Take the drive offline even if redundancy is lost on the array)
must be selected. This option results in a degraded storage pool due to the degraded MDisk,
as shown in Figure 4-8.
Figure 4-8 Degraded MDisk
The IBM Storwize V5000 Gen2 storage system prevents the drive from being taken offline if it
can result in data loss. A drive cannot be taken offline (as shown in Figure 4-9 on page 149) if
no suitable spare drives are available and based on the RAID level of the MDisk, no sufficient
redundancy will be available. Click Close to return to the Internal Storage panel.
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Implementing the IBM Storwize V5000 Gen2 with IBM Spectrum Virtualize V8.1