BIOS and OS  Limits
on Large Hard Disks

Copyright©2004-2005 by Daniel B. Sedory



6 to 64-bit Hex Numbers and Limits
(Take the link for an explanation of the hex math Limits!)

30 GB "Partition" Limit

Windows™ 2000 cannot format a partition greater than 30 GB using FAT32, but it can use a larger FAT32 partition if it was created under Windows 98/Me. Microsoft claims that they designed it that way! No doubt many users will see that as a convenient explanation to cover up a 'bug' they never fixed. For more information, Knowledge Base articles Q277546 and Q184006. Even Windows™ XP has some FAT32 limitations, which are addressed in Q314463 and Q310525. These problems may be altered by SP updates; so check on those as well.

32/33.8 GB Limit

Some computer's BIOS can not recognize HDDs over about 32GB; as a matter of fact, they may even lock-up or hang when you try connecting a larger drive to them! This was our experience with a 'PCChips' M577 Motherboard and a 40GB drive! Most likely this is the same limit problem that Andries Brouwer discusses under Section 2.11 as the 33.8GB limit (65535 x 63 x 16 = 66,059,280 sectors, or 33,822,351,360 bytes). Since our box wouldn't even boot-up normally, we had two choices:

1) To jumper the drive as being only 32GB and then install DDO software allowing the BIOS to see the full size of the drive, or
2) To FLASH the BIOS!  Fortunately, we were able to find a free BIOS source for our motherboard and carry out the procedure without a power supply failure. If there had been one, our M577 Motherboard would have become useless!! And the BIOS chip on this particular Mobo cannot be replaced since it's soldered in place!

Note on BIOS chip Flashing

Today, you may have a computer with a good enough motherboard that it can still restart even if the flash process fails; but this is not something you can simply test for, you must know from the motherboard manufacturer (check both the documentation that came with it and their web site for your exact model). If you do have one of these modern systems, then you can flash your BIOS with any new features you want without worrying about a power failure... as long as you make absolutely sure the new BIOS code is for the exact model of your motherboard and that you have the instructions for how to proceed in the event of a power failure.


Possible 64GB Limit

If you are running Windows™ 98/98SE and want to use an HDD over 64GB, you really should review all the Knowledge Base articles related to these conditions! You can start with this one: Q263044 at the Microsoft Support site, .


The 137GB Limits (Win98, BIOS and even Win2000)

In the recent past, I used to say here: "You can not use Windows™ 98/Me with any drive over 137 GB (it's exactly 128 GiB)! Ever." Turns out that is not really true!!! It was based upon some poor wording found at another site! To be exact, let's quote the applicable text from Microsoft's KB article :

"Operating systems that do not have 48-bit LBA support enabled by default (such as Microsoft Windows 98, Microsoft Windows Millennium Edition (Me), or Windows 2000) that are installed on a partition that spans beyond the 28-bit LBA boundary (137GB) will experience data corruption or data loss."
[ Emphasis added! ]

What this means is: These OSs can be used on a drive larger than 137 GB, as long as the partition(s) they operate from and have access to (for reading or writing) do not span beyond the 137 GB boundary! Therefore, you could create and use a number of Windows 98/Me partitions anywhere on the drive below the point of 137 GB and have no problems. That's not what some web sites seem to imply. Note: first appears to say this applies to the whole drive, not just the size of a single partition nor where it's located; quoting from their first page: "Some versions of Microsoft Windows do not support hard drives greater than 137 GB at all,..." But later on, in a rather confusing page about Windows 98, they give examples of how it can be used with such drives! Apparently this confusion is because so many people create only one single partition, no matter how large a drive they have, which would indeed lead to data loss under Win 98/Me! Read the FAQ at their site for more info.

Many computer BIOS will be unable to recognize hard disks over 137 GB, since they are limited to using only 28 bits to keep track of the LBA sector count (28-bit binaries can hold a maximum value of FFFFFFF Hex, or 268,435,455 sectors; if you multiply that by 512, you get: 137,438,952,960 bytes; thus, the 137GB size limit). NOTE: Since all Partition Table entries use 32 bits (a Hex "Double Word" 4 bytes long) for their Starting and Total Sectors counts, there won't be any problems with the present MBR sectors until HDD sizes exceed about 2,048 GiB! For details on the Hex numbers here, see our page 6 to 64-bit Hex Numbers.

Even Windows™ 2000 (prior to SP3) is unable to use any space on an HDD beyond the 137-GB boundary; until you apply the SP3 (or later) update to it! This means that you will not be able to install an early Win 2000 OS on such drives using the original install CD; unless you create an initially smaller partition underneath that 137-GB boundary! That's a very good reason to purchase Win 2000 install CDs with SP4 already applied.

Some References:

Section 2.11 "Limits" of Andries Brouwer's, Partition Types pages:  [Read all of it!] (Caution: Can be confusing!)

Last Update: November 10, 2005.

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