Insufficient privileges to boot from another disk

[Mac Hammer Fan]

I have to bootable SSD's in my Mac Pro. One with El Capitan and one with Sierra. I wanted to change my startup disk in Sierra to El Capitan but the computer always reboots in Sierra. When I lock the startup disk control panel I get the message that I have unsufficient privileges and I can't change the startup disk to the selected disk.
I have made a screenshot but I can't post it here with the method that I always used in the past. Strange.

 

[Mac Hammer Fan]

I found the solution. Instead of selecting the "Restart" command I selected "Shut Down". And when I started the Mac it booted in El Capitan. Seems to be a bug in Sierra? Never experienced this in the past.

 

[Fishrrman]

Try this.
Try it a few times booting "back and forth".

1. Shut down -- all the way down, power off
2. Press the power on button
3. IMMEDIATELY hold down the option key, and KEEP HOLDING IT DOWN
4. The Startup Manager should appear
5. Select the drive you wish to boot, and hit return.

Does this work when you first boot one drive, then the other, then the first, etc. ????

 

[Mac Hammer Fan]

I don't have a bootscreen with my PC-nVidia Geforce 980, so this doesn't work.
The Startup Manager is invisible for me.
But the problem is solved. If I choose "shut down" instead of "restart" the Mac boots with the correct startupdisk.