When my imac is coming out of a deep sleep such as an hour or more. When the screen wakes and the password screen comes on, it flashes off or black for a quick second then back to password screen. It does not do this during use or a quick sleep just coming out of a long sleep. How do I fix this? Is this a major problem worth taking the imac to an apple specialist? Is it a hard drive failure. Is this normal please help. Imac is a late 2012 27 inch model with fusion drive.