Removing the Subassembly
You need to remove the subassembly only if you are installing or removing the hard disk drive that is mounted next to the power supply.
Follow these steps:
1.Turn the computer so you are facing the front panel.
2.Disconnect the power supply and drive cables from the backs of all the drives installed in your computer, as shown below. Note which cables are connected to which drives so you can easily reconnect them later.