Should be able to disconnect the power and IDE or SATA communications cable from rear of drives, they pop off, do not twist or rock the connector off or you could damage the HD or Cable. Remove 4 retailing screws that hold drive in place. *Some systems require the face panel of the computer to be removed and there are ejector clips that hold the HD into a track, then the HD is removed through the front and replaced through the front. While others have plenty of room to remove the drive by carefully pulling it out of the 3.5" bay towards the inside center of the case paying attention not to hit the memory RAM sticks that are often near by. Sometimes the power cable to the motherboard is also in the way and had to be unplugged if there is not enough slack in it to bend the cable out of the path of the HD being removed. Some systems even have 3.5" bay trays that are removable by removing 1 screw and sliding them out or allowing for them to then swing from a pivot point 90 degrees allowing easier access of hard drive swapping or replacement.
Also to note, sometimes you will need to take the opposite side panel off of your tower to gain access to the 2 screws opposite side that hold the hard drive into the 3.5" bay. Be sure you are performing all of this unplugged so that if you drop a screw onto the motherboard its not a smoke show, as for when powered the motherboard has power to it and its ready for the power on signal to boot. While most of the motherboard is disabled (off) power is still present. And for safe ESD, its best to touch the frame of the computer tower first before touching any computer parts to equalize your bodys extra electron count with that of the computer tower chasis, so that the discharge is a safe discharge and your discharged before reaching into the case.
Good Luck
