My external hard drive wont let me saves files from my Mac laptop

I have a Coolmax 3.5 External Enclosure HD-389-U2 and i'm using it on a Powerbook G4 Mac OS X 10.3.9. When i connect it, it reads the drive just fine and i tested it on my PC and it saved files quickly and easily and when i re-connected back onto my mac, i was able to drag and drop the files i saved from my pc onto the mac but not vice versa. Everytime i try to drag something i want to save onto the hard drive folder from my mac i get a message stating "blank blank blank could not be moved because the "name of my backup drive" cannot be modified". The problem isn't the hard drive cause it worked on my PC and the backup drive was formatted as a FAT32, so what can the problem be then?. Any help would be greatly appreciated.

P.S.
I'm new here, so sorry if this is a newbie question

Did you format the ext HD from the PC or Mac? All I can think to add is to try formatting it on the Mac using MSDOS (FAT32) after you back up all the data on it.

How big is the ext drive btw.