Yes, OS X has a root partition. Look at the link in my previous post. Without going into the multitude of various installation combinations the root partition is defined during installation. Look at your /etc/fstab. Unless you select an existing partition during installation it is not part of the operating system. For installation A the root is partition a, for installation b the root is partition b. You do not want to share root partitions with multiple linux distributions. /root can be any partition on any drive attached to your computer.

When you install multiple distributions on the same system you need to manually create and select the desired partitions which can be sometimes daunting for someone that has never done this before. There are typically 3 installation methods. Totally automatic where the installer will completely overwrite any existing operating system, Creates a dual boot where the installer creates or uses free space on the existing hard drive or something else which is where you can manually create and assign partitions with a specific mount point. I don't remember not being able to select any partition with any mount point.

None of my OS X partitions are different to the others so how can one be a root partition?

My OS X installations do not populate /etc/fstab by default.

@michaelk

Your links did not help me much.

Can you explain why during an install on empty partition B the installer says I cannot use / as a mount point because it has already been used?

OSX is based on BSD, so it's different in many ways from Linux. But I find it hard to believe that it has no root partition.

When you boot Linux, you have to tell the kernel which partition to mount as its root. All the other partitions are mounted by mount, but the root partition is mounted by the kernel. In order to actually function as a root partition, the specified partition must have the expected directory structure on it and basic software (shell, Unix utilities, libc and so on).

You talk about "booting from the other Linux partition". In fact you don't boot from a partition at all. You boot from the bootloader (GRUB or whatever). GRUB needs to know where to find the kernel corresponding to the menu option you have chosen, and the kernel needs to know where its root partition is. Usually the root partition is also the one that holds the kernel, but that's not compulsory. The LFS book, for example, recommends that when you are dual-booting LFS with other Linuxes, you keep all the kernels together on a separate boot partition (though I don't do that). It was also common, in the early days of Linux, to have a separate small boot partition with the kernel on it, because those early PCs couldn't boot from a large disk due to BIOS limitations.

When you are installing, "/" usually refers to the root partition of the Linux running on the installation disc. It therefore has to mount your eventual root partition somewhere else (for example on /mount) in order to copy the files over. But you should always let the installer do that kind of thing by itself. Don't try to mount it yourself unless you are explicitly told to do so.

Linux may contain any number of partitions too. And they may or may not differ from each other (just the files stored). Anyone of them can be root partition and anyone of them can be ignored.
During the install the user should select one to use as root partition, because root partition must exist. But it depends on the user which one is selected.
The root partition usually has a special content which is necessary to boot that OS. Another partition may contain another OS and during boot you can choose which one to boot (and including which partition should be used as root partition).

I may be wrong because we don't have a breakdown of your partitions, but I think your problem is caused not because the install is seeing root on your original setup but because it is already creating it for you on the new setup. Some installs do this.
You have to tell the install exactly what you want and that would mean changing its suggestion for the new setup.

@ hazel

My 'root partition' confusion results from my not being sure if we are talking about 'the' root partition or 'a' root partition. In other words is there a fixed partition which has a root function of some kind or does the term root partition simply apply to the active partition ie the one we are installing into or have booted?

Perhaps I use boot(ed) too loosely. I think of booting into (or from) the OS which is just about to control my computer. I think I am correct in saying that this is done by firmware and/or battery backed RAM calling /System/Library/CoreServices/BootX on one of my many volumes (partitions) with OS X.

I had always assumed that the use of / for a mount point during installation referred to the OS/partition being installed and not the location of the installer or some other location on the disk(s). The more I read the more I think this is probably correct but it does not explain why a stick mounted Ubuntu 9.10 installer reacts the way it does during installation of a second instance on an already defined second primary partition.

I don't see how your final paragraph would work with an installer on ROM as it cannot customise its own /mount. My installer was not on ROM but most installers can work on ROM.

@ pan64

I agree with all you say except that I suspect several of your 'the' should be 'a'.

@ petelq

My only partitions are: Windows, Linux, Linux, Fat 32.

What you say is what I would expect but it does not explain why the second Linux install refuses to accept / as its mount point because it has already been used.

see my post #20, the root partition is (more or less) where the OS is installed.
You can read for example wiki pages about the boot process itself, it is quite complex nowadays. The "final" step is to use a filesystem to run the OS itself. The root partition contains that filesystem.

During installation another OS is working - I mean not the one you install, but another one which will be able to do the installation. Therefore (the currect) / is used by the installer itself and the root filesystem of the target OS is just "under construction".
I have already told you, you misunderstood that message. see post #4.

I don't really understand what do you want to say here.

@ pan64

You said "Therefore (the currect) / is used by the installer itself" but this does not explain why one of the installers options for the mount point is /.

I am not sure how to use quote tags here without the loss of the message history.

When you install from a cdrom or dvdrom, the installation OS boots to a root partition on a ram disk. It creates this root partition itself by copying a partition image with all the essential software onto a ram disk which it has asked the bootloader to create. Individual programs are unpacked onto the ram disk as required. As you rightly point out, the mounting of your final root partition onto which the whole system is to be copied, wouldn't work entirely in rom.

Actually most usb installation images work the same way. Although the drive is writable, it isn't actually written to. Distros that are actually designed to be used from usb have a thing called persistence, where they allow you to store things permanently on another part of the usb drive, as you normally would on a hard drive.

@ pan64

Rather than "several of your 'the' should be 'a'" I should have said "several of your implied 'the' should be 'a'".

@ hazel

Thanks for the RAM disk explanation.

It still does not explain why one of the mount point options is /.

I don't think anyone can answer that unless they sit with you during the install and find out exactly what options this particular installer program is offering and in what sequence. Otherwise it's just too abstract.

Do you have a local Linux user group (LUG) anywhere near where you live?

Is it likely that the first install used a mount point at the root of the stick without any warning and the second install refused to use the same mount point?

I used the same installer several years ago from a CD for a single install and did not notice anything wrong.

I wonder what happens with when an OS either has no mount point or has an incorrect one.

It may not take me long to do both installs again and examine the mount point options in more detail.

My point was, that if you set up the installer to install on your 2nd linux partition (/dev/sda3?) then it will set that up as the / mount point. If you then try to tell it something different or reset it will tell you that the mount point has already been set.
If you want it to be something different you would need to unset the installer's decision and reset your own.
However it seems that you are installing the whole system including your /home on one partition so you don't have any other choice as to where the / mount point is. The rest of the install would just be sub-folders of the root (/).