Problem Sharing External Hard-drive

Hi! Recently my parents bought my sister an external hard-drive for college to hold all her music and such, just as we do at home. So I copied all the music she wanted off our home external hard-drive onto her new one using our Windows XP Desktop. After everything was on it, I connected her hard-drive into her macbook. I can play all the music on the hard-drive, but if my sister wants to move say, her Incubus folder of music already on her pc into the external hard-drive, a window pops up saying that the external hard-drive cannot be edited. I looked at the folder info on the mac to find that all the folders were read only, so i plugged it back into the desktop we own to make all the folders not read only (because I don't know how to do that on a mac). No luck, the same thing occurs. If I try to put things on the external hard-drive it says that the device cannot be edited. What should I do?

 

this worked! thanks bro cheers