I followed Bay 13's web instruction (http://volvospeed.com/850fbreak2.htm) to change front brake disc. Removed caliper, caliper holder, 10mm stud pin. At the step of taking the rotor disc off the hub (between the 2nd and the 3rd picture on that page). However, the rotor disc wouldn't move at all. Used a lot of WD40 to try to lossen it, no difference.
That page says "use a hammer to break the rotor away from the hub". Doesn't move at all. Looks as if the big Hex nut at the center is holding the whole rotor disc in place, but if I try to remove this hex nut, the hub rotates too.

I'm stuck at this step. Anybody has any idea what to do next?

Appreciate it!

I recently changed mine and had no problems. Most likely your problems are caused by rust between the wheel and rotor because at this point there is nothing else holding it on. As I recall, British Leyland always recommended that you use anti-seize compound behind the rotor for this reason. Just keep rotating the wheel and spray holes with WD-40 or PBlaster or such. An airhammer behind the rotor may be helpfull if you have one available.

Don't remove the hex nut, it is not necessary. Use a large ball peen hammer and tap it at 3,6,9 and 12 oclock positions on the inside. It will come off. Mine was tight too. It is not held on at this point other than friction.

5lb BFH and with 2 good raps behind the rotor surface it will come off! Been there, done that! Replacement rotors are not that expensive. FCP Groton has them for under $40 a piece.

Well, spent another 2 days on it, went through one can of Super Liquid Wrench, knocked the rotor at least 200 times with a mallet. Couldn't move it one mm :-(

For V70AWD, '98, there is a round metal sheet behind the rotor (to shield the rotor from dirt from inside of the wheel). This metal shield is held by 2 bolts, one of them can only be reached after you take out the rotor! Only leaves an opening at the location of the caliper that I can hit the rotor on. Besides, there is not a lot of space that allows me to swing the mallet hard onto the rotor. So I don't know how any of you can 'knock it out' unless you have a garage facility that gives you more room to swing your hammer...

I'm going to give it up after 2 frsutrating days... Maybe it's not something for the weekend mechanics at home.