Bless, Boot != Root, and System Startup Fogginess

Question

I've been scouring the Internet, and asking this question in other forums, and either I'm terrible at searching, or the answer is fuzzy and unclear:
How does one configure the root volume? Not the Boot volume, but the volume that the system mounts as "/"?
Configuring the boot volume is easy: one goes to System Preferences-->Startup Disk. But, twice now I've run into this issue: I have Disk A and Disk B. Disk A was the system the OS was installed on. I mirrored it onto Disk B. I went into Startup Disk and told OSX to boot from Disk B. If I yank disk A from the system (a Mac Pro) and boot up, Disk B starts up beautifully. Everyone's happy. BUT if Disk A is in the system, OSX will boot from Disk B as the *boot* disk, but then it'll still make Disk A the *root* disk, meaning if you do a df -k /, or diskutil info /, you will notice Disk A is mounted as "/"! Not what I want.
I looked at bless (the man page is very non-technical and non-detailed), and attempted both:
bless --mount /Volumes/DiskB --setbooot --verbose
and
bless --device /dev/disk2 [the equivalent of DiskB] --setboot --verbose, with no success.
Someone please enlighten me, or point me to some technical documentation.
Thanks!

Answer 1

I tell you the truth, I have no clue as to what is happening in your system. What you are describing
doesn't happen normally in Darwin (OS X).
Normally, the Volume that the system boots from (whichever one it is: A, B, C, D, etc.), gets the root
"/" assigned to it, and only it. All other mounted Volumes get assigned as /Volumes/<A, B, Etc.>.
Darwin is unlike Unix/Linux in that respect. Additional mounted Volumes don't normally get mounted "Unix Style" as /boot, /usr, /usr/local, /swap, etc, however you can force "Unix Style"
behavior if you wish by editing the /etc/fstab file.
You can use the fstab file to mount a Volume on a directory at boot time. The directory must,
of course, be empty and available on the boot volume, to be able to mount a second Volume
as a subdirectory.
Although deprecated, the /etc/fstab mod still worked up until Leopard 10.5.x. I have not tried it in Snow Leopard.
Here is the format in case you wish to try it:
97AC9040-81DD-351B-8675-20C53C555620 /macdata ufs rw 0 2
#<device> <mountpoint> <filesystemtype> <option> <dump> <fsckorder>#
notes: OS X only recognizes the "ufs" *filesystem type* and the "rw" or "none" option.
The last two ( dump and fsckorder ) should always be "0 2", or left off completely.
The Universally Unique Identifier (UUID) must be used to prevent mounting the wrong Volume.
The "none" option is handy to prevent a Volume from mounting at boot time.
OS X does not necessarily mount disks in the order they are connected to the controller,
or even mount them in the same order every time it boots.
Maybe that's what you are seeing in disk Utility (using the list command), OS X will, most of the
time, mount disks in different order when booting different Volumes. disk0 becomes disk1,
disk2 becomes disk0, etc.
Here is a sample diskutil command to mount a volume "Unix Style" manually:
sudo mount -t hfs -o nosuid,-w,-m=775 97AC9040-81DD-351B-8675-20C53C555620 /macdata
#mounts the volume "97AC9040-81DD-351B-8675-20C53C555620" on the /macdata directory#
Hope this "muddies" the water up for you.