some issues about partitions and boot manager in dual boot cases with Windows

Jerry McAllister jerrymc at
Wed Oct 18 23:33:55 UTC 2006


I am having a little trouble following exactly what you are asking,
but will take a shot at some of it, anyway.

By the way, please break your lines at about 72 character length.  It
makes it much easier to read and especially to answer in a text based
Email reader such as many of us use.    Most Email programs allow you
to set this and if yours does not, just hit the ENTER key as your line
gets about that long.  It will help.

>            Hi, I'm actually looking forward to install on my computer a 
> freeBSD edition (very probably PC-BSD).  I found this OS extraordinary 
> after a first experience with a live CD. 

Yup.  It is.

> The only thing that keeps me back is the installation: I got a almost 10 
> (even not more) manuals about freeBSD and none, absolutely none, does not 
> specify the very commun possibility of having a disk already with 2 
> partitions in Windows, not to mention even further possibilities as 
> having 2 disks, from wich one having 2 partitions. 

I guess this is what I do not understand.   Most books on FreeBSD that
I have seen, including the FreeBSD handbook (available online at the
FreeBSD website and is essential reading) ve two possibilities.
The first is installing on a disk that is completely given to FreeBSD
and the second is dividing the disk between Some other OS (usually MS-Win 
of some flavor) and FreeBSD.   None that I have seen say two partitions
in Windows being required (just allowed if desired).

> For instance I have 2 computers, each with 2 SATA disks: on the first, the 
> master, Windows 2000 or XP, and on the other, on the first partition, the 
> other Windows flavor, respectively XP and 2000, the second partition, in 
> both cases, being left for data. Both systems are in dual boot; I decided 
> to have ready (and use) both Windows flavors after a couple of very bad 
> experiences of crushings; thus, in cas of crush, at least on of the 2 OS 
> would be still working allowing me to access the data.

Well, you have to make room for FreeBSD somewhere.  FreeBSD does not
run from a Windows slice.   It requires its own slice.   Note, the term
"slice" which generally corresponds to what the MS-Win world calls a
"primary partition"    BSD Unix uses the term 'partition' to refer to
a subdivision of a slice.    That can at first be confusing, because
sometimes people forget and use both terms interchangeably and they
are definitely not the same thing.

Just a little summary:
In the MS and also the BSD world a disk can be divided in to four
primary divisions which in the BSD world are called slices.   In
each world there are ways of further dividing the slices in to sub-units.
In the BSD world those are called partitions.  In the MS world, they
have something called extended partitions, (but they are not the same).

You need to have at least one slice (primary partition is MS speak) to
install FreeBSD.  That slice will be designated as a FreeBSD type and
MS-Win will no longer be able to talk to it.   Although FreeBSD, which
is smarter, will be able to talk to most of the other MS-Win slices.
(I don't know if it can talk to an extended partition yet or not)

That FreeBSD slice can be all of a disk or just part of it and be 
one of the primary slices which are number 1..4.    If a single disk
has more than one OS, and one of those is some MS-Win thing, then it is 
normally better to leave the MS-Win stuff as first on the disk.  FreeBSD
doesn't care, but MS-Win might skrew up otherwise.  If you are going to
use a whole disk for FreeBSD, then it doesn't matter.

> Now I decided to install the freeBSD on the second partition of the 
> second disk (in each case more than 100 GB) and I cannot find in any 
> manual how, or even if or not, this second partition, of the second 
> disk will appear in Sysinstall Program; normally, it would appear as 
> a "da2" specification, but will it be recognized by freeBSD as "a disk", 
> that is, a 3rd disk ? 

OK.  I think you are saying that, on each disk, you have set up two
MS-Win partitions.   I am presuming by that you mean primary partitions
and not some extended thing.  You are planning on installing FreeBSD in
the second primary partition, which FreeBSD would call slice 2 on disk 1.
That is easily do-able.   

I think SATA disks show up as ad_n.   SCSI disks are da_n where 'n' is
the disk number starting from 0.    In that case your first disk would
look like ad0 to FreeBSD and the second would be ad1.   If there are
two slices on each disk, they would be ad0s1 and ad0s2 on the first disk
and ad1s1 and ad1s2 on the second disk.   It sounds like you want to
put FreeBSD on ad1s2.

If I am wrong on this identifier, then it will just be da0 and da1 as
with SCSI disks.   Just switch the a & d characters in all the commands.

> And if not, do I have to format, allocating the necessary space, in 
> freeBSD (Unix) system files ? This because this second partition of 
> the second disk is already formated in NTFS of Windows ... In this 
> case, do I have to put exactly the measure of the partition (done in 
> Windows) for formating in freeBSD? 

First, the slice will have to be converted to FreeBSD type.  It does
not have to be reformatted though as long as the system is able to see
it as a primary slice.    I think that the FreeBSD fdisk and disklabel
that are used in sysinstall will take care of it.  

But if not, or if you are worried that they won't, then boot the CD and 
select to choose the fixit item from the install menu (past the boot 
selection menu) and then run fdisk on each disk.   If you run fdisk 
without any other flags, it will just come badk and give you information 
about the drive.
So, once you get the fixit disk prompt type:
  fdisk ad0
and then 
  fdisk ad1
and see what it tells you.   It will list information for each 
primary slice.  1..4  and unfortunately, it is once place in FreeBSD
that mucks up the terminology and calls them partitions instead of
slices - I think because the fdisk routine was adapted from MS at
some time in ancient history.

The two lines that say 
  The data for partition n is:
  SYSID nn (OXnnn)......
are the meaningful ones because it tells you what each slice has on
it as far as FreeBSD will know.

Your main MS-Win slice (primary partition) will probably either show 
up as either partition 1 or partition 2.   There may be some diagnostic
piece put on it by the hardware vendor that uses up slice 1 so MS starts
in slice 2, but never tells you.  But, FreeBSD will tell you.

Then, that second slice will show up as either partition 2 or partition 3
depending again on if there is a 'hidden' diagnostic slice on it.
It should say that it is of type (0x07) NTFS and a bunch of other things
if it is that second installation of MS stuff that you want to overwrite
with FreeBSD.    

If there is nothing else on the drive, then slice 4 and maybe slice 3
will show up with nothing on them, just the 'partition' identifier.

If you are worried about that slice being clean and ready for FreeBSD
you can blast it by writing a few blocks of zeros on it using dd.
Lets say it turns out to be partition 3 on the second drive (ad1)
Do the command:
  dd if=/dev/zero of=/dev/da1s3 bs=512 count=32
Be careful with this because if you do it to the wrong place you will
wipe out something you don't want to mess with.   I really think you 
should not need to do this anyway.

So, anyway, sysinstall will give you a list of slices that exist with
space in them and are thus available for installing FreeBSD.   If you
want to install in the 'second' slice of that second disk, it will
be ad1s2 or maybe ad1s3 if there is a 'hiden' diagnostic partition.

>      I ask this because in a forum I 
> noticed that someboyd had some problems after leaving a little space 
> between his 2 partitions of the disk (the first with Windows, the 
> second of Linux). 

I think the problem from that posting (if I remember correctly) was that
he had a little space between slices, but not enough to install FreeBSD
the way he wanted to and was wondering how to find more space.  He wanted
to use a utility called growfs, but that only allows it to grow if there
is enough space right next to a partition, within a slice.  So, he was
stuck with rearranging his disk or adding one to get enough space.  It is
not the same as your situation.  If you intend to use all of that 100GB
in the 'second' slice on the second disk, you don't have to do anything
extra - other than the remote possibility of having to wipe the tables
on that slice using dd as I mentioned above.

> And a last thing, even more worring: the booting and the MBR; do I have 
> to leave the MBR untouched or to install the freeBSD boot manager in MBR? 
> In a Linux manual it is clearly specified, in the chapter about dual boot 
> with Windows, that in this last case - if the Linux boot manager is 
> installed in MBR - than Windows OS (XP or 2000) will not boot anymore ... 
> So, leave the MBR untouched when installing freeBSD or not ?

The FreeBSD MBR can boot the MS-Win systems.   It works fine.  I have
three systems sitting right here in my office that do it with XP and W2000.

You have to tell it to install the FreeBSD MBR on both disks, not just
the one where you are putting FreeBSD.  That is because the MS-Win MBR
is too weak to boot anything besides MS-Win.   So, the way boot works
in this case is:
  The BIOS goes through its boot order looking for bootable media.
    That order is usually something like:  floppy, CDrom, first hard disk
  Presuming you do not have a floppy or boot CD, then it reads the MBR
  from the first (and only the first) hard disk.   The MBR then looks
  for bootable slices on its own drive and lists those and then looks
  for other hard drives that have an MBR and lists those.
  The bootable slices are listed as F1..F4 and any other disks with
  an MBR as F5..Fn, so if there are two bootable slices on the first disk
  (ad0) and an MBR and more bootable slices on the second disk (ad1) then 
  it might look something like this:
    F1  MS-DOS
    F2  ????

  Then, if you choose F5, it will transfer control to the MBR from the
  next disk and that one will look up the bootable slices it has.  Say
  you had two bootable slices on that disk, with the second one being
  FreeBSD, it might look something like:
    F1  MS-DOS
    F2  FreeBSD

You can choose F2 and FreeBSD will boot or F1 to boot whatever OS is
in the MS-DOS slice.

I don't know if the sysinstall will install the MBR in both places.  You
might have to do that by hand in the fixit mode after things are done.
Use  'fdisk -B ad0'    and    'fdisk -B ad1'   for this.

The '????' above comes because FreeBSD uses an honest MBR which occupies
only one 512 byte disk sector as it is supposed to.   That means it doesn't
have much room for extra stuff.  They crammed in recognizable labels for
the identifiers that were know at the time it was first created, and then
make all the rest be ????, including those that came on the scene after
the FreeBSD MBR was created.   Some other MBRs cheat and use more space
under the assumption that it will not really be used by the system.  That
is true in most cases, but is not guaranteed.  It gives them the ability to 
put out more fancy labels, etc at the [small] risk of something not working.

By the way, it goes by the file system type (numerical code) in the slice
partition table.   So, anything that uses a FAT32 or FAT16 will be called
MS-DOS regardless of whether it is Win-95 or XP or Win-2000.   If it is
NTFS, it will display ???? regardless of which version of MS-Win it has, etc.
I think there is room for 255 codes in the slice table.

I think it would be good if the FreeBSD MBR would put the sysid number
it reads up along with the ???? just to help identify it, but maybe that
would take too much code space.

Anyway, MS the recently (relative to the others) added the NTFS file 
system type came in to being after the FreeBSD MBR was created and so it
gets displayed as ????.    If there is a diagnostic slice, it will also
probably get displayed as ????.   

One more thing.  This might be something to deal with at another time
when you decide on its need and maybe are doing a major upgrade.
FreeBSD can read NTSF filesystems, but, so far, it cannot write to them.
For that reason, on systems that have NTFS for their MS system, I have
put an extra FAT32 slice where I could write stuff from FreeBSD and then
read it up in XP or whatever.   That would mean dividing up your 100GB
in to two slices - one small for sharing data and the rest for FreeBSD.
I put that small transfer slice in between the MS and FreeBSD slices
so it comes out as either slice 2 or 3 (if there is a diagnostic slice)
and FreeBSD goes in to either slice 3 or 4.    It works fine and
is only needed if you want to write stuff to the MS system using NTFS.

> Excuse for my long message but I'm a little exasperated and don't 
> understand why this extremly important aspects are not stressed 
> enough in the BSD manuals: how to install the boot (in case of 
> coexistence with other OS, mostly Windows) and how to choose and 
> partition the disks already partitioned ... 

Well, it is pretty well layed out in the FreeBSD handbook, though maybe
not quite as pedantically as I have done above.   That handbook is
important reading.

> I am almost sure that many potential freeBSD users are hold back from 
> installing it because of these capital issues ... 

Some of the understanding comes from reading and then just doing and
seeing how it all works out.   It makes a lot more sense when you try
it and see it happening just as the handbook says.

For example, choosing the drive and slice on which to install; if you
go ahead and run sysinstall, you will see just how it gives you
the choice.   Don't worry, until it says it is going to really do
something, it won't make any changes and you can back out.   So, just
start it up and follow the steps until it says it will wipe things
out and write new stuff and compare what you see with what you need.

Good luck and have fun,


> Thank you very much
> Dana
> _______________________________________________
> freebsd-questions at mailing list
> To unsubscribe, send any mail to "freebsd-questions-unsubscribe at"

More information about the freebsd-questions mailing list