Booting from cd and no /dev/md devices?
First, correct kernel modules are needed.
If configuration uses different raid, then a different raid module should be loaded. Be it a raid 0,5,6 or whatever.
Devices can be made then
mknod /dev/md0 b 9 0
mknod /dev/md1 b 9 1
and so on. As many of them as needed.
From this point on door is open for mdadm create,assemble or whatever is needed.
Creating a device goes like this:
mdadm --create --verbose /dev/md0 --level=1 --raid-devices=2 /dev/sda2 /dev/sdc2
A drive in an Linux md raid array is failed and replaced.
As md raid will not rebuild your array by itself something needs to be done.
Using mdadm, it goes like this:
mdadm - -add /dev/mdX /dev/sdaY
where /dev/mdX is the array that has a failed member, and /dev/sdaX is a partition that needs to be added.
/dev/sda needs to be partitioned before though.
Then it is only a matter of time to see Your arrays being syncronized.
watch -n1 'cat /proc/mdstat'
That array can be in use during that syncronization process.