1. Check to make sure the external hard drive is correctly plugged in.
2. Check to see that the external hard drive is compatible with the computer and device its being used with.
3. Check the computer for any missing drivers that may be connected to the external hard drive.
4. Restart both the computer and the external hard drive.
5. Check the computer for any power management issues affecting the external hard drive.
6. Check the device manager of the computer for any errors.
7. Contact the manufacturer of the external hard drive.