How do I add a new HDD without formating the other older HDD?

I don't think you need to

You can use disk management to change drive letter D to something else, but if you have any programs or data that is expected to be on drive D it will most likely fail after you change the drive letter.

As for formatting the new drive, hook up the power and data, and power up the PC. Go into disk management, right-click the approriate drive and "initialize" it. You can then format and assign a drive letter to it.

there is no risks right?

No, ther is no risk. In disk management, you will see one drive listed as system/boot. Don't mess with that one. The new drive will most likely show up as "raw". This is the one you want to initialize.

sry for late reply dude I was on winter holiday though can u pls write the whole step one by one!! I posted my reply by xpera x10i

Right-click on "my computer" and select "manage".
Select "disk management"

To change a drive letter, right-click on the drive you want to change and select "change drive letter"

Right-click on the drive you need format and select "initialize". It's pretty straight forward from there.

You can copy all of your program files (if any are installed on D) to the "new" d drive.

1) move (rename) old "d" drive to F or something (assuming E is your CD)
2) initialize (partition/format) raw/new drive to D
3) copy files from old d (now F) to new D drive
4) profit

thx