After you plug in a bootable USB device to your MB & still intend to boot from the original HDD with OS, please follow the steps below to adjust the BIOS settings.
1. Hit [Del] during POST to access CMOS Setup of your MB's BIOS.
2. Find "Advanced Setup" --> "Hard Disk Drives" to adjust "1st Drive" to your HDD as below screenshot.
3. Then you can see your HDD is set to the "1st Boot Device" like below. Hit [F10] to save & exit BIOS. System will boot from the HDD.