Though this question is very related (if not the same) to the one you asked here: http://serverfault.com/questions/91302/1tb-harddrive-not-working-in-linux, here's a little tidbit anyway.
What you are experiencing, is very likely Linux trying to mount your new disk instead as your root filesystem instead of your existing disk. This is why using LVM, UUID's or ext3 labels in /etc/fstab is such a good idea.
My guess is your disk is detected as /dev/sda by the kernel, so it tries to set it up as /, but is unsuccessful in that, because the disk is emtpy. To verify this, can you attach the disk to a different SATA or ATA connector (preferably as a slave) and then try again?