even on hard drives and interfaces that support it, s.m.a.r.t. information may not be reported correctly to the computer's operating system. some disk controllers can duplicate all write operations on a secondary "back-up" drive in real time. this feature is known as "raid mirroring". however, many programs which are designed to analyze changes in drive behaviour and relay s.m.a.r.t. alerts to the operator do not function properly when a computer system is configured for raid support. generally this is because, under normal raid operational conditions, the computer is not permitted by the raid subsystem to 'see' (or directly access) individual physical drives, but may access only logical volumes instead.