Section 6-23 of the service manual
Remove the T10 screw from the LS-120 bridge board
Remove the bridge board from the LS-120 drive
Disconnect cables and remove the bridge board, or work around it in the chassis
Disconnect the ribbon cable from the 2.5” IDE HDD (pull gently on each side)
Remove the two T20 screws at the top of the HDD mounting plate
Tilt the plate forward and lift it from the bottom mounts, it will take a lot of fiddling to get it all the way out. It may be easier to remove the PCI cards first, but be very careful of the ribbon cables.
Remove the four T10 screws holding the HDD to the plate
Remove the drive, replace with the new HDD
Follow previous steps in reverse to install the new HDD