OOPS! Raid in linux doesn't use /dev/sda. /dev/sda is an individual hard drive, not the partitions therein. When you did the install and were partitioning the hard drives, didn't you see the option to make raid devices? (Unless the nomenclature has radically changed since I set up my network.)
Go to www.google.com/linux;
search term: /dev/md0
You may have my problem, I have a tendency to read too fast and not in depth. Then I beat myself up for not remembering everything. Take the time to read the "Raid Howto" and then look up other people's explanation of the same subject.
When you have the "Aha!", write your own explanation of it and post it to the web (After you get everything just the way you want it.). Your explanation will be the one that gives someone else the "Aha! Now I get it!" moment. Besides, if you lose your notes--you can always look yourself up on the Web.