The Goal

I was adding a new Seagate Exos X20 (ST20000NM007D, 20 TB) to my Synology DS1821+ running DSM 7.2.2. These drives ship from the factory in 512e mode - they physically use 4096-byte sectors but advertise 512-byte sectors for legacy OS compatibility. Since my NAS and DSM support native 4K sectors, I converted the drive to 4Kn (native 4K) before adding it to the storage pool.

Why Native 4K?

512e is a compatibility shim. The drive’s physical sectors are 4096 bytes, but when the OS writes a 512-byte block, the drive must read the entire 4096-byte physical sector that contains it, modify the relevant 512 bytes, and write the full sector back - a read-modify-write cycle on every write. Native 4K eliminates this: every OS write maps directly onto a physical sector.

Modern Linux, btrfs (which Synology DSM uses internally), and DSM itself all handle 4K sectors natively. For large drives like the Exos X20, 512e exists purely for compatibility with old systems that can’t handle 4K. If your NAS supports 4K - and the DS1821+ does - there’s no reason to leave the drive in emulation mode.

This conversion erases all data on the drive. Do it before adding the drive to a volume.

Prerequisites

  • A blank (new or freshly wiped) drive - the conversion erases everything
  • A USB thumb drive
  • SSH access to the NAS
  • SeaChest_Lite static binary for x86_64 Linux (musl) - available from Seagate’s SeaChest GitHub repository under the ToolBin releases
  • Firmware .LOD file for your specific drive model - available from Seagate’s support downloads for the drive

Copy both files to the root of the USB share (not inside any subdirectory - the path matters for the commands below).

The procedure below follows the guide published on synoforum.com.

Steps

1. Connect and identify the drive

Connect the thumb drive to the NAS (the front USB port works fine), SSH in, and change to the USB share:

cd /volumeUSB1/usbshare

List all devices SeaChest can see:

sudo ./SeaChest_Lite_x86_64-alpine-linux-musl_static -s

Vendor   Handle       Model Number            Serial Number          FwRev
ATA      /dev/sg0     ST20000NM007D-3DJ103    ZVT86D8D               SN03
Synology /dev/sg1     DiskStation             6C5608052080           PMAP
ATA      /dev/sg2     ST20000NM007D-3DJ103    ZVT6M8GH               SN03
JetFlash /dev/sg3     Transcend 64GB          AA00000000000489       1100

The /dev/sg* handles are not stable across reboots - the kernel assigns them in discovery order. Use dmesg to cross-reference by SCSI address instead:

dmesg | grep 512-byte

[   44.517821] sd 10:0:0:0: [synoboot]  245760 512-byte logical blocks: (126 MB/120 MiB)
[346252.143974] sd 7:0:0:0: [sata2] 39063650304 512-byte logical blocks: (20.0 TB/18.2 TiB)
[347010.275382] sd 11:0:0:0: [usb1]  118110208 512-byte logical blocks: (60.5 GB/56.3 GiB)

The 20 TB drive is SCSI address 7:0:0:0. Map that address to its sg node via sysfs:

ls -lh /sys/class/scsi_device/7:0:0:0/device/scsi_generic

drwxr-xr-x 3 root root 0 Jun 27 18:50 sg2

The target drive is /dev/sg2.

2. Update firmware

Flash the firmware before changing the sector size. The synoforum guide treats this as a prerequisite - presumably to ensure the drive is on a build known to handle the format change reliably. Whether your existing firmware version is already sufficient is unclear, so just do it:

sudo ./SeaChest_Lite_x86_64-alpine-linux-musl_static \
    -d /dev/sg2 \
    --downloadFW LongsPeakExosX20SATA-STD-512E-SN03.LOD

/dev/sg2 - ST20000NM007D-3DJ103 - ZVT6M8GH - SN03 - ATA
......
Firmware Download successful
New firmware version is SN03

3. Reboot the NAS

Reboot before continuing. The firmware update doesn’t fully activate until the drive is power-cycled.

After reboot, SSH back in, cd /volumeUSB1/usbshare, and re-identify the device node using the same dmesg | grep 512-byte and ls /sys/class/scsi_device/... method from Step 1 - the node often changes after a reboot. On my machine it moved from /dev/sg2 to /dev/sg1.

4. Verify current sector size and 4K support

Check what sector size the drive is currently reporting:

sudo ./SeaChest_Lite_x86_64-alpine-linux-musl_static -d /dev/sg1 -i | grep Size

    Logical Sector Size (B): 512
    Physical Sector Size (B): 4096
    Cache Size (MiB): 256.00

Logical 512, physical 4096 - that’s 512e. Then confirm the drive supports the 4096 logical format:

sudo ./SeaChest_Lite_x86_64-alpine-linux-musl_static -d /dev/sg1 --showSupportedFormats

 Logical Block Size  PI-0  PI-1  PI-2  PI-3  Relative Performance  Metadata Size
--------------------------------------------------------------------------------
*               512     Y     N     N     N                   N/A            N/A
               4096     Y     N     N     N                   N/A            N/A

The asterisk marks the current format. Both 512 and 4096 appear in the list, so the drive can be converted. If 4096 were missing, you would stop here.

5. Pre-conversion health check

Run a short DST (Drive Self-Test) before committing to the format change. It takes a couple of minutes and catches obvious hardware problems:

sudo ./SeaChest_Lite_x86_64-alpine-linux-musl_static -d /dev/sg1 --shortDST --poll

--poll tells SeaChest to wait for the test to finish and print progress, rather than returning immediately and leaving you to check status manually.

Short DST
    Test progress: 100% complete
The self-test routine completed without error.
Short DST Passed!

6. Convert to 4K native

This step erases all data on the drive. The --confirm flag is required - SeaChest refuses to proceed without the full phrase as a deliberate safety gate:

sudo ./SeaChest_Lite_x86_64-alpine-linux-musl_static \
    -d /dev/sg1 \
    --setSectorSize 4096 \
    --confirm this-will-erase-data-and-may-render-the-drive-inoperable

SeaChest prints a series of warnings and a countdown timer before starting. Once it begins, do not interrupt it - a reset mid-format can make the drive unusable. The command takes a few minutes, then:

Successfully set sector size to 4096

You may also see a warning about being unable to erase the MBR. This is cosmetic - the sector size change itself succeeded. If you later find you can’t write a partition table to the drive, erase the first and last sectors with dd and try again.

7. Reboot and verify

Reboot the NAS again. The new sector size is not visible to the OS until the drive is power-cycled.

After rebooting, SSH back in, cd /volumeUSB1/usbshare, re-identify the device node using the same dmesg + sysfs method, and run the short DST again to confirm the drive is healthy after the format change (same command as Step 5):

sudo ./SeaChest_Lite_x86_64-alpine-linux-musl_static -d /dev/sg1 --shortDST --poll

Short DST Passed!

The drive is now 4Kn. Add it to a Synology volume or RAID group through DSM Storage Manager as you normally would.

What Works

DSM 7.2.2 on the DS1821+ recognized the converted drive without any extra steps. Adding it to a storage pool and creating a volume worked exactly the same as with any other drive.

Caveats

This procedure was done on a Seagate Exos X20 (ST20000NM007D). SeaChest is a Seagate-specific tool; non-Seagate drives would need a different utility. Before converting, always run --showSupportedFormats to confirm your specific model supports 4K logical sectors - not all Seagate models do.