Cant move files from Mac G4 to WD Elements

Hello,
Can anyone who is knowledgeable in this kind of problem help me please..
I have purchased a WD Elements 2TB External Hard Drive and im using it in windows pc, but I also need to back up my files from my Mac G4 10.3.9 version,my Mac can read the files from external but I cannot put files from mac to the external,what should I do?im not kinda good in using mac,please someone help me in this problem??i need to used this external in windows pc & in Mac,is it possible??