That is probably because of Windowz itself. Try right-clicking on My Computer icon and choosing Manage and then Disk Management. Next, be careful which option do you choose on the popup message box! Disk Management window is divided on two parts - volumes list and drives list. Locate your PS2 HDD in drives list and check its size there. If it is not the proper one try to switch it to another mode (like cable select) or fix it to master/slave.
"Only those who try will become." -- Kimahri Ronso