Just in case the hard drive may be giving you trouble run diagnostics on the system. Restart the system and at the Dell splash screen start tapping F12. Run full diagnostics paying special attention to the hard drive results.
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No bootable devices found can be caused by many factors like inappropriate BIOS settings, corrupted MBR and etc. To solve this problem, you can enter BIOS to check whether you set the bootable hard drive as priority. If you boot from a proper device but the error still appears, try to rebuild MBR.