assigning partition letters in dual boot

Discussion in 'Windows 64bit' started by Tom Orle, Dec 31, 2009.

  1. Tom Orle

    Tom Orle Guest

    Happy New year all,

    I'm having an interesting challenge to keep me busy over this long
    weekend. The basic question is: what, on a dual boot W7 system,
    determines how partition letters get reassigned?

    I installed W7 Pro x64 on an existing Win XP Pro 32 bit system as dual
    boot. But first I cloned my existing drive just as a precaution.

    The initial partitions on both 500 GB drives were C (XP boot), D, E
    and a newly created F for W7.
    Install #1: I installed W7 into the F partition and when done I booted
    into W7. As expected it made the F partition the C partition, D and E
    remained D and E and the XP partition became G since somehow the DVD
    drive slipped in as F.

    Install # 2: Two weeks later I wanted to install W7 onto the cloned
    drive. I didn't do anything different than I did during Install # 1,
    but this time when W7 booted up F became C as expected, but the old XP
    partition slipped into the D slot and the old D and E were shifted to
    E and F!! The DVD drive became G.

    My question now is: why did these 2 identical installs come up with
    different partition letter assignments?
    And how can I force the assignments to look like my first attempt
    since I like keeping my data partitions, D & E the same under XP and
    W7? Not that it matters to the system as it works either way, but it's
    less confusing to me since I would always know that certain data is on
    the same partition no matter which OS I'm logged on to.

    It seems that he letter swaps must happen in the boot manager. However
    I checked into BCDEDIT and the boot tab in msconfig, but found no way
    to affect the letter allocations.

    Any ideas and suggestions would be very welcome.
    Thanks ...

    Tom Orle, Dec 31, 2009
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  2. Tom Orle

    Kerry Brown Guest

    It's very hard to determine how Windows will enumerate the drives and
    partitions in advance. Fortunately you can easily change the drive letters
    after the fact. Run diskmgmt.msc then right click a partition or drive to
    change the drive letter. Do not change the letter for the partition that the
    currently running version of Windows is installed on. You should also not
    change the letter for partitions where you have installed programs for the
    currently running version of Windows. If it's just data you should be OK.
    Kerry Brown, Dec 31, 2009
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  3. Tom Orle

    R. C. White Guest

    Hi, Tom.

    When we install Win7 by booting from the Win7 DVD, Setup has no idea what
    letters have already been assigned, so it follows its own rules for the
    initial assignment. First, it assigns the letter C: to whichever partition
    we choose to "install" it in. This becomes the Win7 "boot volume". Then it
    assigns other letters, starting with D:, usually assigned to the "system
    volume", if that is not the same partition as the boot volume. (This rule
    changed with Vista; WinXP and prior assigned C: to the system volume, then
    the next available letter to its own boot volume, if different.) After
    installation, we can use Disk Management to change letters on all volumes
    except the boot volume and, if different, the system volume. The Status
    column of Disk Management shows us which partition has the Boot status and
    the System status; the two "status" labels may or may not be on the same
    partition. (For these "backwards" definitions, see KB 314470, Definitions
    for system volume and boot volume )

    But when we boot into an existing Windows installation, then run Win7 Setup
    from that desktop, Setup can see the drive letters already assigned by that
    existing OS and it will use the same letters, even for the boot and system

    So, when we add Win7 to an existing Windows system to create a multi-boot
    system, we can either have it "inherit" the existing letters (by running
    Setup from the existing desktop) or have it assign "C:" to whichever
    partition we install it into (by booting from the DVD).

    Note that Windows (any version) is just as happy running from X:\Windows as
    from C:\Windows. Most users develop a "drive C: mindset" early in their
    computing experience and have trouble adapting to having Windows on any
    other partition - but Windows itself has no such hangup. If you want your
    drive letters to be consistent as you reboot between different Windows
    installations, then boot into the first system, assign the letters you want,
    and then run Setup from there to install the second system - and the 3rd and
    4th OSes, if you want to use that many. Only ONE can say it's on Drive C:,
    but neither Vista nor the others will care if Vista is on Drive V:. And
    nobody cares if we skip letters and choose some that are easy to remember; I
    like to leave gaps that I can fill in later if I get a new thumb drive or
    other device.

    Win7 has added a new wrinkle by creating a small (100 MB) partition to use
    as the System Volume, but that happens only when Win7 is installed on a
    computer without an existing OS. I've read about this, but haven't run into
    it yet, since I was already multi-booting before installing Win7, so it
    simply updated the startup files on my existing System Partition.

    BCDEdit uses drive letters, but it cannot change them. We use Setup to
    assign letters initially and Disk Management to reassign them - except for
    the system and boot volumes.

    R. C. White, CPA
    San Marcos, TX

    Microsoft Windows MVP
    Windows Live Mail 2009 (14.0.8089.0726) in Win7 Ultimate x64
    R. C. White, Jan 1, 2010
  4. Tom Orle

    R. C. White Guest

    Hi, Tom.

    "Drive" letters are like shifting sands. Especially when dual-booting.
    Please don't rely on them. Windows pays very little attention to them.

    When dual-booting, OS #1 cannot read OS #2's Registry, so it has no idea
    what letters OS #2 might have assigned. So what is Drive X: in WinXP might
    be Drive H: in Win7 - or vice versa. To make the drive letters consistent
    between the two OSes, you need to run Disk Management in each OS and assign
    the same letters in each of them.

    But there is a BIG fly in this ointment: Letters for the System Volume and
    Boot Volume are assigned by Setup when each OS is installed and cannot be
    changed (by us mere mortals) after installation - except by running Setup
    again, which means re-installing.

    The terms "system volume" and "boot volume" are VERY important in
    understanding drive letter assignments - and the definitions are backward
    from most users' expectations. As often said, we BOOT from the SYSTEM
    partition and keep the operating SYSTEM files in the BOOT volume. Carefully
    read KB 314470, Definitions for system volume and boot volume . Until you get these
    definitions firmly in mind, dual-booting is not going to make any sense.

    WinXP Setup assigns the letter C: to the System Partition, then assigns
    other letters according to its built-in algorithm. In the typical system,
    with the first partition on the first HDD holding both the System and Boot
    volumes, this means that the whole shebang is in Drive C:. Most users never
    get past this basic arrangement and never need to really understand drive

    Vista Setup assigns the letter C: to its own Boot Volume - wherever that may
    be - even if it is the 3rd partition on the second HDD. Whichever partition
    you choose to install Vista in will become Drive C:. That means that,
    unless you install it into the System Partition, the System Partition will
    have to be assigned a different letter: Drive D:.
    R. C. White, Jan 1, 2010
  5. Tom Orle

    R. C. White Guest


    Sorry for the double post. This second one was an earlier draft that I
    meant to delete, not to send. Please ignore it. :>(

    R. C. White, CPA
    San Marcos, TX

    Microsoft Windows MVP
    Windows Live Mail 2009 (14.0.8089.0726) in Win7 Ultimate x64

    R. C. White, Jan 1, 2010
  6. Tom Orle

    Tom Orle Guest

    I'm aware of the latter. That's why I installed a piece of software,
    like Open Office, twice - once in XP and once in W7 just to make the
    respective registry aware of the softwares location. despite having
    only one copy of Oo on drive D.

    So when drive D got shifted to E I ran into problems. My choice was
    to either edit the W7 registry and change all drive references to Oo
    from D to E or change the drive letters .
    But I lucked out - since both drives were identical copies of each
    other and I'm a proud owner of Acronis' backup software and was smart
    enough to make a backup copy after the first sucessful install, I
    copied/restored just the MBR onto the drive with the shifted letters
    and everything fell in place.
    This makes me assume that, when booting into W7 on a dual boot
    system, it's the boot manager that does the assignments of drive
    letters. If I look at bcdedit, it shown W7 on drive F and /ntldr on C,
    but after boot W7 is C and the C boot drive containing XP became G!

    But thanks to all for your educational insights - I learned a lot.

    Happy New Year!!

    Tom Orle, Jan 1, 2010
  7. Tom Orle

    Sardine Guest

    Sardine, Jan 20, 2010
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