Most probably the 500gb disk (p4).
If you are able to get to desktop, Go to RUN and type compmgmt.msc.
In the side tab, Check STORAGE -> Disk Management. You can check which drive has the OS installed.
P1 seems to be ur ASUS DVD drive.
Select the disk which has the OS installed.
P2- 2TB HDD
P4- 500Gb HDD
P5- 2Tb HDD
@mods Please move thread to OS Subforum.