The 3ware RAID controller has the ability to detect if the system has been shutdown via the standard shutdown mechanism of the operating system (clean shutdown). If the system loses power or is reset without going through the normal shutdown procedure, it is possible that the data on a redundant unit may be out of synchronization. The unclean shutdown detection will detect this case and force the unit to enter the verify state.
To prevent unclean shutdowns, always go through the normal shutdown procedure. It is also recommended to use an uninterruptible power supply (UPS) to prevent unclean shutdowns due to sudden power loss.