What list of things would you suggest that I do?:

1. Check that the external hard drive is plugged into a power source and the correct port on the computer
2. Ensure that the external hard drive is compatible with your OS
3. Check that the external hard drive is in working condition
4. Ensure that the external hard drive is powered on correctly
5. If needed, install the necessary drivers for the external hard drive
6. Run diagnostics on the external hard drive
7. Reinstall the OS, if needed
8. Check if the external hard drive appears in Device Manager
9. Reconnect the external hard drive and retest