TrueNAS showing "Mixed Disk Capacities" warning when disks are same size

TrueNAS General
1

I am using TrueNAS 24.10. Added a new disk to one of the pools today. It initially had a 4TB Seagate IronWolf disk, now added a Exos disk.

NAME                                       SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP    HEALTH  ALTROOT
mediasonic-media-pool                     3.62T  2.05T  1.57T        -         -     0%    56%  1.00x    ONLINE  /mnt
  mirror-0                                3.62T  2.05T  1.57T        -         -     0%  56.6%      -    ONLINE
    f9350321-ac49-4590-821a-0564e11e8bf8  3.64T      -      -        -         -      -      -      -    ONLINE
    60ae6998-051b-44ee-bb8d-f3c28255851d  3.64T      -      -        -         -      -      -      -    ONLINE


disk 1:
sudo gdisk -l /dev/sdc
GPT fdisk (gdisk) version 1.0.9

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.
Disk /dev/sdc: 7814037167 sectors, 3.6 TiB
Model: Expansion Desk
Sector size (logical/physical): 512/4096 bytes
Disk identifier (GUID): 016F31C0-5A54-45D5-BE5E-720D3DEFCA28
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 7814037133
Partitions will be aligned on 2048-sector boundaries
Total free space is 3692 sectors (1.8 MiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048      7814035455   3.6 TiB     BF01  data

disk 2:
sudo gdisk -l /dev/sde
GPT fdisk (gdisk) version 1.0.9

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.
Disk /dev/sde: 7814037168 sectors, 3.6 TiB
Model: ST4000NM000A-2HZ
Sector size (logical/physical): 512/512 bytes
Disk identifier (GUID): 4279AE0A-08AD-475C-B2E1-AFCABA42299E
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 7814037134
Partitions will be aligned on 2048-sector boundaries
Total free space is 3693 sectors (1.8 MiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048      7814035455   3.6 TiB     BF01  data

As you can see everything about the disks are same except for model and sector size (512 vs 4096)

Is this what causes the UI to show this warning?

2

Look again.

Disk /dev/sdc: 7814037167 sectors
Disk /dev/sde: 7814037168 sectors

3

One sector difference…

4

Great minds think alike!

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5

It’s possible that the disks might differ in size by the slightest amount. This could be a difference of a few MB or even a single sector!

I would check to see if your disks differ in size by comparing their total sector count.

If this is the reason, don’t forget to mark my post as the solution. :white_check_mark:

6

Thank you for the quick responses guys. I went back in dmesg and sure enough I have this

[103952.418070] GPT:Primary header thinks Alt. header is not at the end of the disk.
[103952.418200] GPT:7814037166 != 7814037167
[103952.418289] GPT:Alternate GPT header not at the end of the disk.
[103952.418377] GPT:7814037166 != 7814037167
[103952.418465] GPT: Use GNU Parted to correct GPT errors.

even though gdisk verification didn’t say anything. After gdisk x and e looks like we have both disks equal!

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