Security Analytics, with 8.2.2 installed and U840 storage arrays (also known as VA084 or ME4084) attached may have unused drives marked as "Available". These are intended to be used as hot spares. The system may not have enabled "Dynamic Spares" allowing these available drives to be used as spares. When a drive that has been allocated to a virtual disk fails, the intended hot spare will not be used and the virtual disk will not be fully redundant until the failed disk is replaced.
Initial installed release : 8.2.2
The U840 storage array has a web interface which allows it to be configured, updated, or monitored. The Dynamic Spares Capability can be enabled in the menu under System -> Actions -> Advanced Settings. Select the check box to the right of the line "Dynamic Spares Capability". Select Ok or Apply.
This will allow the system to use any available drive, not assigned to a virtual disk, to be used as a hot spare. When a disk fails, the system will proceed to rebuild the data missing from the failed drive onto the now hot spare. After several hours the virtual disk once marked as "Degraded" will become fully redundant.
Also, this is a good time to check dell.com/support for any available firmware updates for the storage array. These can sometimes be done on a lightly loaded but active system. Using the Asset Tag or serial number from the array, you can check Drivers and Downloads for the latest firmware.