am i guessing right your external was once connected to a windows pc
as it would explain why you cant write and just read the files as its formated in ntfs i suppose
and snow leopard has disabled the feature of writing to ntfs simple because ntfs is a proprietary file format from microsoft
there are several ways to enable read and write functions
easier would be to safe the files somewhere and reformat the external in Mac OS Extended file format or if you happen to still having a windows pc then Fat 32 file format