That's normal behavior - OS-X can read, but not write to NTFS. It can read/write FAT32 (and, of course, Mac OS Extended (journaled)).
You can use macfuse with the ntfs plugin.
Can someone tell me what format the MAC uses for external hard drives? I have a drive that is NTFS on the PC and i can read it on the MAC but can not write to it.
Thanks
...just use FAT32 drives and you won't have a problem!