Internal HDD write protected!!!

i have an external hardisk wd500 and an ibm laptop. the problem is when i copy/cut a file from my lappy to the wd500 it says disk is write protected connot copy, use another disk. but when i copy files from wd500 to lappy, it does and also i can delete files in my lappy. another thing when i cut a file from my wd500 and paste in my lappy it actually just copies it. i think my internal hdd is write protected somehow,how do i remove it???