What happens if you disconnect the power supply cord to the hard drive?

If you disconnect the power cord to the hard drive, when you turn the computer back on, the hard drive will not be operational. Additionally, if you leave the IDE (or SATA) cable connected when the power cable is disconnected, the system will take longer to POST because it will try to identify the device connected, but it will not be able to. If it is the only hard drive you have in your system, the computer won't boot up because your operating system is on the hard drive.