Boot Partition above 16Gb line?

  cga 12:53 23 Aug 2004

As a follow up to my earlier thread click here I beleive that my problem is that my main boot partition is now above the 16Gb line.

I am guessing that the MBR cannot nativly address an active partion that starts above 16Gb.

I am starting a new thread to get an accurate description of the issue.

Can anyone confirm or deny this, or point me to a reference on the limits of an MBR? I have looked without success so far?

  bremner 13:27 23 Aug 2004

I may be misreading what you have said but can you confirm that you have a Primary Partition partitioned into three volumes P1,P2 and P3 and that the O/S is on P3

  cga 13:50 23 Aug 2004

Yes - that is correct. More accurately P2 & P3 both contain bootable OS's but P3 contains my normal OS and is set to active.

Previously it booted correctly like this but, after I extended the partition sizes I get the message 'missing operating system' when booting normally. P3 used to start around 12Gb but is now at about 17.5 Gb.

When I boot via Bootmagic with P3 as the chosen partition then no problem.

I have done extensive reading but can find no reference to a limit but I have something in the back of my mind that I have seen this somewhere.

  bremner 13:55 23 Aug 2004

I have never heard of the 16GB limit

But it was my belief that a primary partition should not itself be partitioned. click here and from reading Scott Mueller.

An extended partition can have up to 24 sub partitions.

Hopefully someone with greater knowledge will post.

  cga 14:05 23 Aug 2004

Sorry - misunderstood the wording of your question.

You are correct - a primary partition cannot be further partitioned. You can have up to 4 primary partitions on a disk. One of those can be an extended partition which can contain sub partitions.

What I have is:

Primary 1 - 2Gb FAT16 Non bootable - Dos Tools

Primary 2 - 15.5Gb NTFS Bootable - XP Safety System (Standalone)

Primary 3 - 15.5Gb NTFS Bootable (Active) - XP Production.

Ext 1 - Code

Ext 2 - Data Backup

I realise that my home system is slightly unusual and guess noone has tried this before

  Fruit Bat /\0/\ 14:14 23 Aug 2004

Not sure why you are having problems at 16GiB

When drives larger than 504 MiB began to appear in the mid-1990s, many system BIOSes had problems communicating with them, requiring LBA BIOS upgrades or special driver software to work correctly. Even after the introduction of LBA, similar limitations reappeared several times over the following years: at 2.1, 4.2, 8.4, 32, and 128 GiB. The 2.1, 4.2 and 32 GiB limits are hard limits: fitting a drive larger than the limit results in a PC that refuses to boot, unless the drive includes special jumpers to make it appear as a smaller capacity. The 8.4 and 128 GiB limits are soft limits: the PC simply ignores the extra capacity and reports a drive of the maximum size it is able to communicate with.

  cga 14:25 23 Aug 2004

I understand your point.

The complete disk is visible and accessible. It just wont boot into P3 without the help of Bootmagic (normally I disable bootmagic except for problem solving).

My guess is that the boot pointer in the MBR has a bit limit equating to 16Gb-1 but, from my reading so far, I have been unable to verify that.

I thought that there would be a dozen regulars who were able to confirm this instantly but, in the absence of that, I will have to solve this impirically by fiddling with partition sizes with Partition Magic. Just take a long time to proove that it is this and no other change that resolved the problem.

I will report on my findings in a couple of days.

  cga 17:04 28 Aug 2004

After extensive tests eliminating all other variables I can report that my supposition is correct.

1. You CANNOT successfully boot from an active partition that starts above the 16Gb line.

2. You CAN successfully boot from the same partition when the start is moved just below the 16Gb line.

3. Use of a boot manager (in my case Bootmagic) DOES allow you to boot from a partition starting above 16Gb that cannot be booted natively.

  bremner 17:10 28 Aug 2004

Where did you discover this or was it just from your own observations.

  bremner 17:16 28 Aug 2004

Where did you discover this or was it just from your own observations.

  cga 17:18 28 Aug 2004

I exchanged my disk and increased the partition sizes as I copied them on

I normally boot into P3 (long story - my safety system).

When I tried to boot it said 'Missing Operating System'

Tried with bootmagic and it booted OK.

Tried again without bootmagic - same failure.

Looked for what the differences were and P3 had moved just above the 16GB line - no other difference. So I formed a theory and asked if anyone could confirm.

Finally I spent a few hours doing controlled tests and was able to confirm myself.

This thread is now locked and can not be replied to.

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