I did manage to install new hard drive as master and old drive which came with the DVR as slave by using power splitter and squeezed in 2 drives by leaving one drive over the board with non-conductive packaging that comes with drive underneath the drive i.e. between the drive and board to avoid any short circuit, however my only fear is if it over heats and damages the board over the period. Otherwise DVR has recognised both the drives and working OK for now.
Thanks for all the advise.