deepakmenonk :
Do you think it will fit in? Because WD HDD is 9mm height and Seagate is only 7.5mm
Replacing a 9mm drive with a 7.5mm drive is usually not a problem. Most laptop HDD caddies require you to screw the HDD to the caddy, so it won't flop around despite extra 1.5mm of space.
In the event that your laptop doesn't use a caddy or the caddy doesn't hold the HDD firmly in place, you can stick some pieces of open-cell foam (like a sponge) onto the HDD with some tape. Avoid any holes clearly marked "do not cover," and try to mount the foam close to the corners of the HDD where the case is less likely to flex from the pressure.