|If the RAID is internal it would be a good chance the operating system is installed to it. Evidently, you are using the RAID array for storage/ backups. |
There are many factors that can make a RAID array fail. If any one of the drives gets out of spec on any important spec, for instance.
What type of RAID array are you using and how many drives are included? Are all the drives identical?
See the link below if you are not sure.