I have Debian Squeeze in a RAID 1 on 2 physical 2 TB disks. Is it possible in the future to add two physical disks and extend the RAID 1?
I've removed the md-based instructions. They didn't take into account the striping issues of RAID0 and may have led to data loss. The LVM solution listed below is better anyway.
If you're using LVM, this is a lot easier. The process is similar, but you use LVM to span the two RAID1s. Follow this tutorial:
Using LVM would indeed be the easiest way. Even if you don't use LVM yet you may consider using it with the new disks. That means that you cannot transparently increase the size of your current volume but have to use mount points for space extension.
Having four disks you can do both RAID-0 and RAID-1 simultaneously (if all are the same size or you are willing to do without some of the capacity). But using RAID-0 requires copying your data.
Without RAID-0 and without LVM you can to this: You create a second RAID-1 with the new disks. If the old RAID is md0 and the new one is md1 then you can create a combined device:
Not a general problem but I don't know whether the boot scripts are clever enough to get that done. If you don't use your whole disks for your RAID-1 but single partitions in them (e.g. md0 for /boot, md1 for / and md2 for data) then it's not problem as you can use your own boot script to set up your "two level RAID" if necessary.