Can I open and put my own HDD without voiding warranty ?
Unfortunately no, it's unlikely. From MSI's warranty information:
" 1.4 Circumstances where free maintenance service is not provided
In any of the following conditions, whether the Product is within the free maintenance time-limit or not, the Company may refuse to provide the maintenance service, or may request you pay related parts and labor charges, as well the to and from shipping charges;
You are unable to present the Product's warranty card, purchase receipt or invoice for the prove of Product still within the warranty period;
The warranty card has been altered or its date is hard to recognize;
The Product is not acquired from an authorized distributor of the Company or is acquired from illicit sources;
The complete unit or parts of the Product has gone beyond the warranty period;
Any breakdown or damage caused by non-compliance of regulations stated on the user manual, or improper packaging, storage and use;
Any breakdown or damage caused by installation, repair, modification or removal (breaking mark, damage or loss of the product compliance label) performed by service centers or personnel not authorized by MSI;
Any product breakdown or damage caused by use of parts not certified by MSI's original factory;
* The serial number pasted on a complete unit or the parts is broken, defective or tally with the mainframe;
Any breakdown or damage caused by accidental factors or man-made reasons (including computer virus, moving, compression, scratch (scrape), hit, crash, high temperature, high humidity, water inflow, use of inapplicable voltage, non-factory battery and transformer, stain, corrosion, etc.);
Any breakdown or damage caused by use of pirate software;
Any scrape (scratch), liquid leak, crack, etc. on the LCD screen surface;
Any breakdown or damage caused by natural disasters and human calamities (such as earthquake, fire, riot, etc.)"