The most likely cause for a system attempting to boot from an optical drive despite BIOS settings is a bootable CD-ROM disk present in the drive. Removing the disk should fix the issue.
If a user reports that after configuring the BIOS to boot to the hard drive first, the system still tries to boot from the optical drive, the most likely cause is the presence of a CD-ROM disk in the optical drive. The BIOS is set to check for a bootable drive in the order specified by the user, and if a bootable disk is found in the optical drive, the system will attempt to boot from it regardless of the boot order settings. Therefore, removing the disk from the optical drive should resolve the issue.