It may be that your BIOS is set up to look for 'other bootable devices' such as an external USB device, first in it's boot up order.

When it doesn't see an OS on these 'other devices' it usually goes to the next device in line such as a ROM drive or the hard drive.

You can try detaching all external devices, and then going into your BIOS when you boot, set the BIOS back to all 'default' settings and then reboot.

This may work.

Good luck.