Interesting. To me, such a thing could occur because GRUB does not assign device names such as /dev/sda to drives. The operating system does that. GRUB relies on the BIOS to determine its device designations such as (hd0). At the time GRUB is going about the business of booting the kernel, that BIOS order is all that really matters because the operating system hasn't started. Since it is not unheard of for operating system device names and GRUB drive designations to get sort of mixed up, maybe the addition of your new drive resulted in the operating system reassigning device names, but the BIOS drive order with the new drive was such that it did not result in a misconfiguration of grub.conf. And thank goodness for partition labels and/or UUIDs in /etc/fstab, I guess.
Where you might have some problems with all of this is if you now needed to re-install GRUB in the master boot record and were to use the grub-install command. The grub-install command always consults the device.map file if the --recheck option is not used (the device.map's only purpose), and it probably still has /dev/sda mapped to (hd0). You can either remember that fact or change the device.map file to match the new reality now in your head. It's what the device.map file is for. It does not have any effect on routine booting whatsoever.
I guess if you decide that you don't like what has happened, you could try playing with controller cables or udev rules.
Last edited by stoat; 14th February 2009 at 05:30 AM.