To improve performance, the 3ware RAID controller features caching layer firmware. For write commands this means that it acknowledges it has completed a write operation before the data is committed to disk. If the 3ware RAID controller can not commit the data to the media after it has acknowledged to the host, this message is posted.
Typically, the Cache Flush Failed notification would be an indication of a catastrophic failure of the drives in the unit, such as loss of power to multiple drives in a unit.