I am wondering if anyone has encountered this before, possible bug maybe, that causes 2TB drives or smaller to show partition format as MBR even when it is GPT?

All the drives in my array show up as GPT, and then I tried adding another drive (this is my only drive smaller than 3TB), but no matter how many times I format it, zero out the partition etc, it just always end up showing as MBR under disk settings in the UNRAID web UI. I've been at this for days and have honestly lost track of everything I have tried, and I think I need a thread to track this so I can hopefully make some progress.

Here is the 2TB disk info from gdisk that shows up as MBR...

Partition table scan:

MBR: protective

BSD: not present

APM: not present

GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): p
Disk /dev/sde: 3907029168 sectors, 1.8 TiB

Model: ST2000DM008-2FR1

Sector size (logical/physical): 512/4096 bytes

Disk identifier (GUID): 596C198C-80D0-430E-B669-C316003C3FF9

Partition table holds up to 128 entries

Main partition table begins at sector 2 and ends at sector 33

First usable sector is 34, last usable sector is 3907029134

Partitions will be aligned on 64-sector boundaries

Total free space is 93 sectors (46.5 KiB)

Number Start (sector) End (sector) Size Code Name

1 64 3907029071 1.8 TiB 8300

Command (? for help): i

Using 1

Partition GUID code: 0FC63DAF-8483-4772-8E79-3D69D8477DE4 (Linux filesystem)

Partition unique GUID: F47F6310-EC83-4180-A13D-3586216B9B2B

First sector: 64 (at 32.0 KiB)

Last sector: 3907029071 (at 1.8 TiB)

Partition size: 3907029008 sectors (1.8 TiB)

Attribute flags: 0000000000000000

Partition name: ''

And here is an example of one of my 3TB drives in my array that shows up correctly as GPT like all of the others...

Partition table scan:

MBR: protective

BSD: not present

APM: not present

GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): p

Disk /dev/sdg: 5860533168 sectors, 2.7 TiB

Model: WDC WD3001FFSX-6

Sector size (logical/physical): 512/4096 bytes

Disk identifier (GUID): 2C1F32DC-5714-4D4F-BAFB-8DD15956E1B6

Partition table holds up to 128 entries

Main partition table begins at sector 2 and ends at sector 33

First usable sector is 34, last usable sector is 5860533134

Partitions will be aligned on 64-sector boundaries

Total free space is 30 sectors (15.0 KiB)

Number Start (sector) End (sector) Size Code Name

1 64 5860533134 2.7 TiB 8300

Command (? for help): i

Using 1

Partition GUID code: 0FC63DAF-8483-4772-8E79-3D69D8477DE4 (Linux filesystem)

Partition unique GUID: 0E6FBCC9-0F3E-487C-8D88-048984AAE86A

First sector: 64 (at 32.0 KiB)

Last sector: 5860533134 (at 2.7 TiB)

Partition size: 5860533071 sectors (2.7 TiB)

Attribute flags: 0000000000000000

Partition name: ''

Any ideas?

this is not a bug, and nothing is actually wrong with your disk. What you’re seeing is a UI labeling quirk + historical partitioning behavior, not a mismatch between how the disk is really partitioned and how it’s being used.

What your gdisk output proves (and this is the key point)

For both disks, including the 2 TB one, gdisk says:

Found valid GPT with protective MBR; using GPT

That means:

• The disk is GPT

• It has a protective MBR (this is normal and required by spec)

• The filesystem is Linux-native (0FC63DAF…)

• Partition alignment is correct (starts at sector 64)

• There is no functional MBR partitioning in use

From a ZFS / XFS / Unraid data-integrity standpoint:

*There is zero difference between the 2 TB and 3 TB disks.

Why the Unraid UI says “MBR” for ≤2 TB disks

This is the part that trips people up.

1. GPT vs MBR is not required for ≤2 TB disks

Historically:

• MBR works perfectly fine up to 2 TB

• GPT is required above 2 TB

• For years, Unraid defaulted to:

  • MBR for ≤2 TB

  • GPT for >2 TB

Even today, Unraid does not require GPT on disks ≤2 TB.

So the UI logic effectively does:

“Disk ≤2 TB? MBR is acceptable → label it MBR-ish”

Even if the disk actually has GPT metadata.

2. Protective MBR confuses simplistic detection logic

A GPT disk always contains:

• A protective MBR at LBA 0

• This exists to stop old tools from overwriting the disk

Some UI checks don’t distinguish between:

• “MBR-only disk”

• “GPT disk with protective MBR”

gdisk does distinguish.
The Unraid UI often does not, especially on small disks.

So it sees:

• An MBR header

• Disk size ≤2 TB

• And labels it MBR, even though GPT is present and active

---

3. Previous formatting history does not matter here

You mentioned zeroing, reformatting, etc.
That’s not the issue.

Your output proves:

• GPT header exists

• Backup GPT exists

• GUIDs are valid

• Partition table is intact

Unraid is not reading stale metadata — it’s just displaying a simplified classification.

---

Why your 3 TB disks always show as GPT

Because they must.

There is no ambiguity:

• MBR cannot address 3 TB

• UI logic must say GPT

• No fallback labeling is possible

So the UI is accurate by necessity for >2 TB disks.

---

Important: what this does NOT affect

This has no impact on:

• ZFS

• XFS

• Parity

• Alignment

• Performance

• SMART

• Future expansion

• Data safety

ZFS/XFS sit inside the partition, not in the partition table logic.

Bottom line

• Your 2 TB disk is GPT

• The protective MBR is normal

• The Unraid UI label is cosmetic

• This behavior has existed for years

• You are not corrupt, misaligned, or misconfigured

But if reusing in unraid yo should zap and let unraid format the disks...

sgdisk --zap-all /dev/sdX

sgdisk -g /dev/sdX

10 hours ago, awschluk said:

possible bug maybe, that causes 2TB drives or smaller to show partition format as MBR even when it is GPT?

This is not a bug; it has always been like that with Unraid, any devices up to 2TB will use MBR, any devices >2TB will use GPT.

On 1/5/2026 at 5:00 AM, JorgeB said:

This is not a bug; it has always been like that with Unraid, any devices up to 2TB will use MBR, any devices >2TB will use GPT.

That doesn't seem correct because as gdisk shows they are using GPT.

On 1/4/2026 at 6:19 PM, awschluk said:

I am wondering if anyone has encountered this before, possible bug maybe, that causes 2TB drives or smaller to show partition format as MBR even when it is GPT?

All the drives in my array show up as GPT, and then I tried adding another drive (this is my only drive smaller than 3TB), but no matter how many times I format it, zero out the partition etc, it just always end up showing as MBR under disk settings in the UNRAID web UI. I've been at this for days and have honestly lost track of everything I have tried, and I think I need a thread to track this so I can hopefully make some progress.

Here is the 2TB disk info from gdisk that shows up as MBR...

Partition table scan:

MBR: protective

BSD: not present

APM: not present

GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): p
Disk /dev/sde: 3907029168 sectors, 1.8 TiB

Model: ST2000DM008-2FR1

Sector size (logical/physical): 512/4096 bytes

Disk identifier (GUID): 596C198C-80D0-430E-B669-C316003C3FF9

Partition table holds up to 128 entries

Main partition table begins at sector 2 and ends at sector 33

First usable sector is 34, last usable sector is 3907029134

Partitions will be aligned on 64-sector boundaries

Total free space is 93 sectors (46.5 KiB)

Number Start (sector) End (sector) Size Code Name

1 64 3907029071 1.8 TiB 8300

Command (? for help): i

Using 1

Partition GUID code: 0FC63DAF-8483-4772-8E79-3D69D8477DE4 (Linux filesystem)

Partition unique GUID: F47F6310-EC83-4180-A13D-3586216B9B2B

First sector: 64 (at 32.0 KiB)

Last sector: 3907029071 (at 1.8 TiB)

Partition size: 3907029008 sectors (1.8 TiB)

Attribute flags: 0000000000000000

Partition name: ''

And here is an example of one of my 3TB drives in my array that shows up correctly as GPT like all of the others...

Partition table scan:

MBR: protective

BSD: not present

APM: not present

GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): p

Disk /dev/sdg: 5860533168 sectors, 2.7 TiB

Model: WDC WD3001FFSX-6

Sector size (logical/physical): 512/4096 bytes

Disk identifier (GUID): 2C1F32DC-5714-4D4F-BAFB-8DD15956E1B6

Partition table holds up to 128 entries

Main partition table begins at sector 2 and ends at sector 33

First usable sector is 34, last usable sector is 5860533134

Partitions will be aligned on 64-sector boundaries

Total free space is 30 sectors (15.0 KiB)

Number Start (sector) End (sector) Size Code Name

1 64 5860533134 2.7 TiB 8300

Command (? for help): i

Using 1

Partition GUID code: 0FC63DAF-8483-4772-8E79-3D69D8477DE4 (Linux filesystem)

Partition unique GUID: 0E6FBCC9-0F3E-487C-8D88-048984AAE86A

First sector: 64 (at 32.0 KiB)

Last sector: 5860533134 (at 2.7 TiB)

Partition size: 5860533071 sectors (2.7 TiB)

Attribute flags: 0000000000000000

Partition name: ''

Any ideas?

On 1/4/2026 at 10:09 PM, bmartino1 said:

this is not a bug, and nothing is actually wrong with your disk. What you’re seeing is a UI labeling quirk + historical partitioning behavior, not a mismatch between how the disk is really partitioned and how it’s being used.

What your gdisk output proves (and this is the key point)

For both disks, including the 2 TB one, gdisk says:

That means:

• The disk is GPT

• It has a protective MBR (this is normal and required by spec)

• The filesystem is Linux-native (0FC63DAF…)

• Partition alignment is correct (starts at sector 64)

• There is no functional MBR partitioning in use

From a ZFS / XFS / Unraid data-integrity standpoint:

*There is zero difference between the 2 TB and 3 TB disks.

Why the Unraid UI says “MBR” for ≤2 TB disks

This is the part that trips people up.

1. GPT vs MBR is not required for ≤2 TB disks

Historically:

• MBR works perfectly fine up to 2 TB

• GPT is required above 2 TB

• For years, Unraid defaulted to:

  • MBR for ≤2 TB

  • GPT for >2 TB

Even today, Unraid does not require GPT on disks ≤2 TB.

So the UI logic effectively does:

Even if the disk actually has GPT metadata.

2. Protective MBR confuses simplistic detection logic

A GPT disk always contains:

• A protective MBR at LBA 0

• This exists to stop old tools from overwriting the disk

Some UI checks don’t distinguish between:

• “MBR-only disk”

• “GPT disk with protective MBR”

gdisk does distinguish.
The Unraid UI often does not, especially on small disks.

So it sees:

• An MBR header

• Disk size ≤2 TB

• And labels it MBR, even though GPT is present and active

---

3. Previous formatting history does not matter here

You mentioned zeroing, reformatting, etc.
That’s not the issue.

Your output proves:

• GPT header exists

• Backup GPT exists

• GUIDs are valid

• Partition table is intact

Unraid is not reading stale metadata — it’s just displaying a simplified classification.

---

Why your 3 TB disks always show as GPT

Because they must.

There is no ambiguity:

• MBR cannot address 3 TB

• UI logic must say GPT

• No fallback labeling is possible

So the UI is accurate by necessity for >2 TB disks.

---

Important: what this does NOT affect

This has no impact on:

• ZFS

• XFS

• Parity

• Alignment

• Performance

• SMART

• Future expansion

• Data safety

ZFS/XFS sit inside the partition, not in the partition table logic.

Bottom line

• Your 2 TB disk is GPT

• The protective MBR is normal

• The Unraid UI label is cosmetic

• This behavior has existed for years

• You are not corrupt, misaligned, or misconfigured

But if reusing in unraid yo should zap and let unraid format the disks...

sgdisk --zap-all /dev/sdX

sgdisk -g /dev/sdX

Fantastic response! Thank you for being so thorough, you covered every angle, and every possible follow up question. So many hours of life wasted because of a UI mislabeling quirk haha!

9 hours ago, awschluk said:

That doesn't seem correct because as gdisk shows they are using GPT.

Not sure what you mean; if a device up to 2TB is formatted by Unraid, it will use MBR, not GPT.

On 1/20/2026 at 2:47 AM, JorgeB said:

Not sure what you mean; if a device up to 2TB is formatted by Unraid, it will use MBR, not GPT.

Have you even bothered to read the response provided by bmartino1? And if you have, are you saying this individual is incorrect in full or in part?

"That means:

• The disk is GPT

• It has a protective MBR (this is normal and required by spec)

• The filesystem is Linux-native (0FC63DAF…)

• Partition alignment is correct (starts at sector 64)

• There is no functional MBR partitioning in use"

Hope this makes it more clear: this applies to any 2TB or smaller device:

If it is added as a new blank array disk formatted by Unraid it will always be formatted as MBR.

If the disk had a previous GPT partition and you add it to a new array, like a new install or after a new config, then the existing GPT partition will be used

If it had a previous GPT partition and is then formatted by Unraid for a rebuild, it will be reformatted to MBR.

if you just format an existing GPT disk, without rebuilding, it will keep the GPT partition.

For the cases where Unraid formats the disk MBR, if some utils still show GPT info, that may be from a previous partition/layout.

Post the output from fdisk -l /dev/sdX for that disk.

In my case, that can only be true for drives less than 2TB, not 2TB drives.