Sometime the loading bar doesn't appear and the boot up procedure stops
When my MAC start, normally appear the Apple Symbol and in the same time a bar under below in progress until the system remain ready. Sometime the bar don't appear and the procedure stop. I have to switch off the system and restart 1 o 2 times before to have the PC ready.
[Re-Titled by Moderator]
iMac 27", OS X 10.11