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.