External Harddrive

Hi,
I am a new mac user (only just purchased my Macbook 2weeks)
I have a Macbook and running Mac OS X 10.5 Leopard,
When i plug my external harddrive, I can move stuff to the Mac however if I try to move stuff which is on the Macbook to the external harddrive, it gives me an error message- it says the device cannot be modified.

If i plug the external harddrive onto my PC i can move stuff either way.

Is there a setting which i need to change?
Please help.

Thanks

Yes, it's a formatting issue. The external drive needs to be formatted as FAT32 for Macs and PCs to be able to read/write to it.

I found a nice freeware program for PC to change your harddrive format:
Its called EASEUS Partition Master 4.0.1 Home Edition

Easy to use, you can change the format of your harddrive with out formatting it.