Hi,
Thank you for your reply.
Please check that the connection between the external hard drive and the computer is strong and complete. Also try to connect the drive to a different USB port and check if the issue happens or not.
As you are getting the Tone every half an hour. Please follow the below link to perform clean boot.
http://support.microsoft.com/kb/929135
Note: A clean boot is performed to start Windows by using a minimal set of drivers and startup programs. This helps eliminate software conflicts that occur when you install a program or an update or when you run a program in Windows 8.1.
Please refer to the section “what is next when I have a Clean Boot Environment?” in the above provided link.
Once you are done with the troubleshooting please refer to the section “How to reset the computer to start normally after clean boot troubleshooting” in the above provided link.
Please reply us back if the issue still persist.