Solution to Synology NAS Error: GPT PMBR size mismatch (239649 != 245759) will be corrected by write.

readme.md

This issue has been there since many years. It can be found on some forums. Searching links from Google https://www.google.com/search?q=Synology+GPT+PMBR+size+mismatch+%28239649+%21%3D+245759%29+will+be+corrected+by+write.&newwindow=1

I personally enabled 2 pcie-nvme SSD disks as Read-Write cache RAID1 on Synology NAS. Then I disabled them safely from DSM web UI (Storage Manager / Storage / Select your Volume), then I found some error from terminal via command fdisk -l. After I removed the 2 SSDs, the error was still there.

Error: GPT PMBR size mismatch (239649 != 245759) will be corrected by write. reported by fdisk -l via SSH to NAS.

GPT PMBR size mismatch (239649 != 245759) will be corrected by write.
Disk /dev/synoboot: 120 MiB, 125829120 bytes, 245760 sectors
Disk model: DiskStation
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: RANDOM_UUID_XXXXXXXXXX

Device         Start    End Sectors Size Type
/dev/synoboot1  2048  67583   65536  32M EFI System
/dev/synoboot2 67584 239615  172032  84M Linux filesystem

The NAS still boots and the RAID volumes still functions well.

NAS: DS423+

OS version: DSM 7.2.2-72806 Update 3

The earliest post about this error was on 19th Oct 2017 from here https://www.synology-forum.de/threads/partitionstabelle-size-mismatch.89351/ The user updated DSM from version 6.1.3 to 6.1.4.

Another recent discussion in Jan 2025 is here https://www.synology-forum.de/threads/gpt-pmbr-size-mismatch.137411/

The response from Synology Support is here

Bitte beachten Sie, dass diese Meldung kein Fehler ist, sondern lediglich anzeigt, dass die Größe der Partitionstabelle (PMBR) und die tatsächliche Größe des Geräts nicht übereinstimmen, was auf die Änderung der von uns verwendeten Komponenten zurückzuführen ist. Dies ist ein erwartetes Verhalten und hat keinen Einfluss auf die Funktionalität Ihres NAS. Sie können diese Meldung getrost ignorieren, da sie keine Probleme verursacht und keine weiteren Maßnahmen erforderlich sind.

Translation in English

Please note that this message is not an error, it simply indicates that the partition table size (PMBR) and the actual size of the device do not match, which is due to the change in the components we are using. This is expected behavior and does not affect the functionality of your NAS. You can safely ignore this message as it does not cause any problems and no further action is required.

Leave it alone if you don't want to fix it

Just leave it alone the volume will still work. If you reboot the NAS, the error will still be there but has no effect with the volumes.

Another recent search from reddit SSD Read/Write Cache Failure led to Volume 1 Crashing https://www.reddit.com/r/synology/comments/99e132/ssd_readwrite_cache_failure_led_to_volume_1/

The user tried fdisk -l

GPT PMBR size mismatch (239649 != 245759) will be corrected by w(rite).
Disk /dev/synoboot: 120 MiB, 125829120 bytes, 245760 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 9799A768-219D-4ED6-8517-8DC40B712849
Device Start End Sectors Size Type
/dev/synoboot1 2048 67583 65536 32M EFI System
/dev/synoboot2 67584 239615 172032 84M Linux filesystem

If you really want to fix this issue, here's the solution

Solution - Fix it using parted

Sources from https://community.synology.com/enu/forum/17/post/95467

Enter via ssh. make yourself root with: sudo su - Then execute: parted Now inside parted app use the command: print all Parted will detect the corruption and ask you about fixing it with options Fix/Ignore. Use the command: Fix Reboot synology. Have fun.

The discussion here

Similar issue and Solution using parted

https://community.synology.com/enu/forum/17/post/95467