Partition one large hard drive into two - use one for TM, the other for archives or storage. You will want to partition the drive because TM will keep eating up space until it runs out. If you limit the amount of space it can use, you can keep more space on the HD for your family photos archive to grow. The danger in one HD, however, is that if you are backing up the family archive to the same drive as TM, if the drive goes to hell, so do your archives.