My guess would be that the FreeAgent HD is preformatted to Windows-compatible. With my MacBook, I have a Windows partition that doesn't allow me to copy files directly between the Mac and Windows partitions.
Like the previous posters, use Disk Utility to reformat to Mac-compatible and the disk permission issue should be fixed.
Like the previous posters, use Disk Utility to reformat to Mac-compatible and the disk permission issue should be fixed.