Ubuntu :: System Automatically RAID Itself?

When I installed this system (Xubuntu 9.04 x64) several months ago, I had two identical SATA hard drives, but I didn't do a RAID1 mirror then because I didn't want to wipe out my old OS (FreeBSD) on the second drive in case I needed to retrieve something from it. So I installed Xubuntu on the first drive (sda) and for all that time, the second drive (sdb) has been running but unused, and fdisk showed that the FreeBSD partition was still there.

A couple weeks ago, my separate backup server failed, so as a short-term backup I did a 'dd if=/dev/sda of=/dev/sdb' to make an exact copy of my Xubuntu install on drive A upon drive B (the former FreeBSD drive), then mounted /dev/sdb1 on /mnt to make sure it worked successfully. So far so good, but I still didn't set up any RAID stuff.

Several days later, I needed to reboot because of a security upgrade, and when I logged out, the GUI froze up. Thinking not too much of it, I restarted the system, and it came up fine. But the next day, I discovered that I was missing several days worth of email and recent files. In fact, everything had been reverted back to a state from several days earlier --- I think from the day I copied the first drive to the second one. Files I created in the interim were gone, and files I deleted in the interim were back. It was as if the first drive was 'restored' from the second one without my knowledge.

Doing some testing now, I find that if I create a new file on /dev/sda1 and then mount /dev/sdb1, the file also exists on sdb1. It's as if they're acting as a RAID1 mirror, without my telling it to do so. Could it just decide to do RAID1 because it sees there are two identically partitioned drives? That seems dangerous. And if they were really in a RAID mirror, why would it let me mount them separately? It's very strange.

I don't mind if it's suddenly decided to do RAID, but I want to make sure it's not going to 'restore' a more current filesystem from an older one again, if that's indeed what happened.