looking back the spec, it is indeed have dvd-rom. so your are in luck. you can buy this type of HDD Caddy and replace it in the current dvd-rom..So you will have SSD and HDD for the extra storage. but make sure the hdd caddy can support your laptop.

Yes , i plan to put the old HDD at my DVD-Rom compartment . Since 120gb is rather small , if teh drive is full , can i choose to install the apps and progammes on hdd ? P/s : I dont play games , so i dont have big game files

Sure you can, when installing anything, just choose custom install and change the installation path. you will be good to go

If wanna keep the mechanical HDD as well...So we close the OS from the mechanical HDD into the ssd...when boot up (in BIOS) we set the priority boot to select the sdd, after boot in successful then format the old OS partition at the mechanical HHD

The process correct?

There is a Restore to Factory OS button in ASUS laptop... I Click that and restore a blank OS in HDD...
Then use EASUS, if not mistaken, then Clone HDD OS(primary)->SSD(Secondary).
Then Swap HDD with SSD, so my new SSD is primary.
As there is not enough for SSD, HDD and DVDROM to co-exist in the laptop. I have taken out HDD tp be an external Harddisk

As bios, there is only one harddisk there...

Mine is HP laptop, got m2 slot, so plan to put in the M2 sata3 ssd and keep the 1TB HHD as well...

mine are old Asus..
ur HP is new laptop? M2 slot sound advance...lol

yup new...