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I’m trying to partition a RAID disk with DISKPART. The format is MBR. The capacity in RAID is 6.7 TB when I’m starting out.

The symptom I’m seeing is that after I create my relatively small primary partitions suddenly the free capacity plummets.

I’m running these commands and I’d expect to have still around 6.5 TB free after but what DISKPART reports is that only about 2TB are left (see screenshot below).

Can someone explain where my free space is going?

select disk 0
clean
convert mbr
create partition primary size=500
format FS=NTFS LABEL="System" QUICK
assign letter=K
active
create partition primary size=100000
format FS=NTFS LABEL="OS" QUICK
assign letter=C

Screenshot of diskpart output below:

enter image description here

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  • Most HDDs and RAID volumes present an LBA sector size of 512 bytes. MBR with 512 byte sectors is limited to 2 TB volumes. You should use GPT.
    – Bob
    Feb 8, 2015 at 8:50
  • Ok, I was somewhat aware of this but the issue is that if I create an extended partition from this point still I can't use the full capacity. My next question would be: is the extended partition also limited to 2TB?
    – bfloriang
    Feb 8, 2015 at 8:52
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    It doesn't matter if the partition is a "primary" one or not. Those are all under MBR. Using MBR at all will not work. You need to switch the entire partition table to GPT. (Once you're on GPT, there is no distinction between a primary or extended partition. GPT has such a large partition count limit that all partitions are the equivalent of "primary".)
    – Bob
    Feb 8, 2015 at 9:13

1 Answer 1

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The likely cause of your issue is the use of a MBR partition structure. MBR disks, assuming 512-byte logical sector size (typical for both HDDs and RAID volumes), are limited to 2 TB per disk.

There are some methods to increase the MBR size limit to 4 TB - however, these are typically not well supported (especially in Windows) and hackish at best. Another option is to create RAID volumes under 2 TB each, and keep them separate.

The best solution is to switch to the GPT partitioning scheme. GPT can support volumes literally a billion times bigger than MBR. Additionally, they can have a much larger max partition count: 128 in current implementations in Windows.

The one significant caveat on Windows is if you wish to make the disk bootable. Windows requires UEFI-boot to boot off a GPT disk. If the disk is for data only, then this is not an issue.

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  • Indeed my use case is such that I'd have the disk to be bootable as I'm restoring from a Backup of a machine with a SAN storage volume attached (bigger than 2TB) this is overall problematic. I'll have to see how I can work around the limitations. In any case, thanks for the explanations!
    – bfloriang
    Feb 8, 2015 at 15:17

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