17,1 iMac, 10.12.6, I have two external backup drives. 1 external drive has a bootable partition w 10.12.6. Somehow my iMac decides to restart from this external partition, rather than my main Sierra fusion drive. How can I remind the iMac to start from its fusion drive, rather than this external partition without holding OPTION. It's like the iMac has forgotten what is the main hard drive.