If both drives are on the same IDE channel, try reseating the IDE cable. If that doesn't work, try replacing the cable.
Be sure all IDE devices are set correctly as Master or Slave, or use Cable Select. However, I have seen the Cable Select feature fail, so try setting the drives manually. In Cable Select, the connector at the end is set as the Master, and the one in the middle is the Slave. You will have problems if one is manually set and the other is set for Cable Select, and the manually set drive is on the wrong connector. I have even seen problems on both IDE channels when one is set incorrectly. Check the settings on your hard drive and any other IDE drives, as well.
If you are sure all your drives are set correctly, the IDE controller driver for your motherboard could be corrupted. Try reloading the IDE drivers, either from your CD-ROM, or check the manufacturer's site for driver updates.