Ubuntu does system disk mirroring right
When we started installing Ubuntu 10.04 systems with our standard mirrored system disk setup, we noticed that it asked us a new and (in my opinion) very stupid question: did we want the system to boot if only one of the two sides of the mirror were there? Of course we said 'yes', since that's part of why we're mirroring the system disk in the first place. Despite its sillyness this question was already an improvement over 8.04, where the installer defaulted to a 'no' and you had to edit the grub settings by hand to change this.
What we didn't notice at the time was what else the installer was doing with mirrored system disks. To wit, Ubuntu now installs GRUB on the second drive, as well as on the first one.
This is an important thing to do, because it's what makes your system
bootable even if you lose (or pull) your entire primary drive. In
the past it was a step that we had to remember to do by hand (with
an appropriate peculiar incantation), which
means that it was sometimes forgotten and so some of our systems have
mirrored system disks but could not actually reboot if they lost the
primary drive. Now all of our Ubuntu 10.04 machines have this handled
automatically for us, which is great and also exactly what a system
should do if it detects that
/boot is mirrored across multiple drives.
Before I started looking, I was going to confidently assert that this was new behavior in Ubuntu 10.04. However, it appears likely that it's also in Ubuntu 8.04 and we just didn't notice; I've checked a few of our 8.04 machines where I'm reasonably certain that we didn't install GRUB on the second disk by hand, and they have GRUB boot blocks.
(Similarly, my just-installed Fedora 15 home machine has a GRUB boot block on the second drive and I'm completely sure I didn't install it by hand, so it looks like Fedora 15 is also smart enough to do this.)
On a side note, it's surprisingly hard to notice changes like this if you don't consciously check for them when you're working out your install procedures for a new distribution release. Our install procedure has always called for installing GRUB by hand on the second drive, so of course we carried that forward into the 8.04 and 10.04 install instructions. Even when this step got accidentally omitted on specific machine installs, we don't normally pull primary drives and do a test boot on the secondary drive. So it took a chain of circumstances that caused us to boot a system on the second drive in a situation where we didn't think we'd set up GRUB on the second disk, and then testing this by installing a test system, deliberately not doing that manual step, and trying to boot the system with just the second drive.
Sidebar: why the Ubuntu installer's question is stupid
The ostensible reason for having an option to not boot if you have a degraded mirror is because this risks data loss if you don't fix it. However, my personal feeling is that almost everyone who is choosing to mirror system disks is doing so in a situation where they would rather have the system continue to operate even with degraded mirrors; people who care that much about data loss are rare, and even then the Ubuntu question is an incomplete solution to the problem.
(Nothing stops the system if your mirror degrades while the system is running, and I think this is the far more likely case.)
I don't object to there being an option for this behavior, but I don't think this is worth a question during installation. If you find that 90% of your audience answers a question one way, stop asking the question and just let the 10% who need a different answer change it by hand afterwards.
(This suggestion is inapplicable for things that can't be changed afterwards, but this is not one of them.)