*** UPDATE ***
I have now heard back from a technician that had been called out most weekends because of problems with the NVR/RAID. Following the suspension of the consistency check, he has not needed to attend once for this problem.
Following a recent NVR Shadow failure using a AVAGO MegaRAID card, engineering found unacceptable delays in writing to the RAID. This culminated in the virtual drive becoming detached from Valerus and the subsequent loss of cameras and recording. It transpires that the RAID was doing a consistency check at the time of the failure. Although the consistency check can be configured with different settings the default runs weekly and can take 24hrs or so to complete. During this time the RAIDs i/o performance is affected by a 30% drop (default) and with a heavily loaded system could cause performance to drop below what Valerus can cope with.
According to my research this feature is probably not needed in the CCTV environment where 100% data integrity is not essential when weighed up against the possible complete failure of the recording.
At any rate, a manual scan can be started during a maintenance period to find/clear any errors on the Virtual drive when it is less likely to cause a issue on site.
Switching off the Consistency Checking can be done through the RAID console of by using the simple tool attached. Simply copy to the desktop and run the bat file as Administrator to check the status, enable checking or disable checking.
NOTE: This in no way suggests that this MUST be done in all circumstances but if you have had an NVR issue on this type of equipment it may just solve the problem.