If you need to initialise a raid1 volume with linux software raid but you currently only have a single disk available, use this:

mdadm --create /dev/md1 --level 1 --raid-devices=2 missing /dev/hda1

You can then mkfs /dev/md1 and then mount and use it as normal. When your second disk is available you can add it into the raid with this command:

mdadm --add /dev/md1 /dev/sda5

Now the raid will begin to sync, cat /proc/mdstat to check the sync progress