Here are some possibilities:
1. If you're using an IDE/PATA drive, ensure no pins are bent on the drive; likewise for the mobo.
2. If you're using IDE/PATA, ensure your BIOS is set to Manual or Auto, under the IDE Channel settings.
3. If you're using SATA, try swapping ports on the mobo. Also try swapping cables. To ensure you're using a known-working cable, use one from a known-working drive.
4. Finally, ensure you've connected the power cable to the ODD as well.