I wanted to try out Software RAID on my new server box. I already had done the Linux install, however and I hadn't set up RAID at that time. Fortunately, I found this Software RAID HOWTO which describes how to add an additional disk and then mirror an existing filesystem. I decided not to mirror my root partition; I didn't think it was necessary. I did set up the partition tables to be exactly the same. That leads to some wasted space, but it made me feel safer. My original filesystems were ext3 (not ext2 as in the document), but I had no problems. Even resize2fs worked perfectly.