My iMac Keeps on try to boot on my external Blu-ray disk drive

My iMac Keeps on try to boot on my external Blu-ray disk drive and not the hard drive. Therefore when I turn it on, it boots up and then it go pass the sound and the apple logo come up on screen but then it frezzes and noing happens; even if I hold down the power button.

System Software Overview:

Boot Volume: Macintosh HD

Boot Mode: Normal

It should not boot off the Blu-ray read/wrigt.

iMac from 2013 Q1

Blu-ray is LG read/wrigt for Blu-rays, HD DVDs, DVDs and CDs pluged in by USB