Hi,

I have a nas / server running Slackware64-13.37 with two raid arrays, one with 3x500GB disks and the other with 2x2TB disks. I also have 1x80GB hdd which contains the os.

I found a spare 250GB hdd from my closet and want to set up raid1 array from os hdd and that spare drive.
I know that I can't make use of the extra 170GB of space but I don't care as long as I have working installation on the other disk if that 80GB hdd breaks.

How can I set up that kind of raid1 without losing any data?

My boot loader is lilo.

Yes you can...

Personally I'd have a look around see if you can get a matching 250Gb drive and create an Array out of that pair and then just rsync the OS across to it.

But you can partition the 250Gb drive to match an identical 80Gb partition and create the array (Even though it will be degraded to begin with). Then rsync the OS to it from the original drive and edit the new fstab to make mdx /. After that you'd shut down the machine, remove the 80Gb drive and boot from a boot disk but into your new /.

You should then be able to reinstall Lilo (after editing the config) and reboot the machine. Once you are happily running with / on your degraded RAID1 you can then partition your 80Gb drive and add it to the Array.

As I mentioned earlier, getting a second drive would be the safest method as you'd always have the 80Gb drive to fall back on.