Jwhy: I've managed to get the format to Mac OS Extended (Journaled) and then I can add files onto the drive on my mac, but then the drive isn't recognised on Windows 7.... I think I need to format the drive to MS_DOS (FAT32) but that isn't an option when in the disk utility on the mac? My options are Mac OS Extended (Journaled), Linux, and Mac OS Extended.
No, the external is not the only HD and I'm not booting off the exernal.
Jwhy: Maybe some context will help the cause... In essence, I've got the MAc and a PC, the Mac has a heap of files on it (music, photos etc) that we want to put on the exernal so we can use them with the PC. So, I think I need to format the hd to a type that is both readable and writable on the Mac and PC (FAT32 I believe) but as above, FAT32 is not an option...