[wplug] Large drive/old (small) bios.

[David Tessitor]

>From everything I've read and from my own experience, as soon as your OS takes
over from the bios there should be no problem, provided the OS can recognize the
large drive.  Linux should be able to recognize it, but NT needs to be using
later service packs and I assume the same for W95.

----

ONE CAVEAT:  From my experience, there can be problems with some partitioning
programs.  For example, regardless of what the OS may see, Partition Magic does
not work correctly unless the bios sees the whole drive.  PM seems to be able to
set up partitions within the area which the bios recognizes, but it can't do
anything for the rest even though PM displays the correct drive size on its
screen.  (At least version 5 has this problem; version 6 is out now and I don't
know if it corrected the problem).

Fdisk, however, under Linux works fine for large drives, BUT only under the
general ("m") menu, NOT under the expert ("x") menu (at least as of a few months
ago).  I don't think this bug has anything to do with whether the bios sees the
whole disk or not.  For some reason fdisk doesn't seem to recognize anything
beyond 1024 cylinders under the expert menu (unless there's a new version just
out that has corrected this).  So, if you try to check things under the expert
menu, they won't necessarily show up right; or if you try to reset or change
certain things under it, they can get all screwed up.  Fortunately, this problem
doesn't seem to exist under the general menu, which enables a simple work
around.

SCENARIO:  Using the general menu selections, you can delete a partition and
recreate it or break that space into two or more.  Or, if you have unassigned
space that is physically before an existing partition, you can also put one or
more partitions in it.  You can do any of this within and beyond the 8.4 Gig
which the bios recognizes.  So far, so good, except for the numbers assigned to
the partitions.

If you've ever tried this on a smaller drive, you may have noticed that the
number assigned such newly created partitions by Linux fdisk will not be
consistent with their physical order.  It has nothing to do with size, it's just
that whenever a partition is deleted, fdisk simply moves up the partition
numbering of all the others after it (e.g. when you delete the third partition
on a five partition disk, fdisk reassigns the former forth and fifth partitions
to be numbers 3 & 4).  However, each newly created partition is added to the end
of the table, where, regardless of its physical order, it is assigned the next
available number (e.g. if you now recreate that #3 partition which was just
deleted, it will instead be assigned #5, even though it is physically in the
middle and is in every other way the same).  The same thing happens if the first
partition you create is at the end or in the middle of a drive or if one already
exists with open space in front of it -- any new partition created in the empty
space is "logically" numbered with the number following the previously highest
numbered partition.  (Doing so seems to make some sense, as it would not screw
up the fstab assignments in an existing installation whenever a new partition is
added.)

Since it easily possible to have hdb1 located in the middle of the drive while
hdb3 is at the beginning and hdb2 is at the end (or any other combination),
things can get rather confusing.  Therefore, it's usually better to have the
logical numbering consistent with the physical, so under the expert menu there
is a simple command that is supposed to reassign the table numbers of all the
partitions to their physical order.  It appears to do this successfully, but it
also seems to hit a critical snag with partitions past the 1024 cylinder
boundary, causing all such partitions to be assigned the same
cylinders/sectors.  (As far as I can determine, this wasn't corrected with the
latest round of distros, either.)  Of course, this is not a very workable
situation!

If you try to use the expert menu and the reorder command, but you don't catch
the address problem and inadvertently rewrite the table, you're up a creek.
Unless you've copied down the beginning and end cylinders of each partition, you
could loose everything in all of your post-1024 cyl. partitions.  (I did find a
repair program yesterday that is designed to determine the actual physical
layout and correctly reconstruct a corrupted partition table.  It's probably not
100% successful, but it could be a big help.  I'll post it to the list when I
find where I put the link.)

PRESENT WORK AROUND:  If after creating all the partitions you want, you look at
the table and see they are not sequentially addressed, you can correct it by
simply deleting the the first partition which is out of order [d, <enter>, prtn#
<enter>], then immediately reconstructing it [n, <enter>, <enter>, <enter>].

If you had completely partitioned the entire disk so there's no free space left,
then there's nothing to worry about -- you'll automatically re-enter the correct
address numbers for the deleted/reconstructed partition while moving its logical
order to the end of the table (i.e. it takes on the highest partition number).
-- If, on the other hand, you had unassigned space remaining at the time you
started the reordering, the default addresses for the re-created partition may
not be the same as for the deleted partition; in this case, you should keep an
eye on the defaults as they are displayed and simply enter the right number when
necessary.

Now that you've moved the first out of order partition to the end of the table,
check the new order with [p, <enter>].  Next, repeat the delete/create process
again for the next partition in the list which has an address that is out of
order.  Continue it until all the partitions are reordered correctly. -- NOTE:
There are some situations where it might be quicker to selectively
delete/re-create something other than the first out of order partition, but this
method does get the job done.

Now, when the order is correct, write the partition table [w <enter>].  NOTE:
You can also write out the partition table before doing the reordering.  Then if
you later make a mistake in the reordering, you need only exit without writing
[q <enter>].  You can then restart fdisk and begin reordering without having to
layout the partitions from scratch again.

FINAL STEP:  You'll need to reset the partition types for any non-ext2
partitions.  You can check the available partition types and their designation
numbers from the general menu (I think its "l"), then use the fdisk command to
set the partition to the appropriate type.

Aside from these partitioning issues, there shouldn't be any problems of which I
am aware from operating with a bios that does not recognize your whole disk.

Dave

============

Matt Hughes wrote:

> Would there be any adverse effects of letting my bios think that it is not
> really the 46 GB drive that it is, but the 7 or 8 GB drive that it wants to
> believe that it is?