"Update DNG Preview & Metadata" corrupts Linear DNGs on SMB Network Volumes (Synology NAS)

**Description of Issue:**

After updating to Lightroom Classic 15.4.1, executing the "Update DNG Preview & Metadata" command on a newly generated Linear DNG (created via HDR Merge, Panorama, or Enhance) living on an SMB network volume causes immediate file corruption. The file structure truncates, a black circle with an exclamation mark (!) appears, and the Develop module throws a "File appears to be unsupported or damaged" error.

This issue does not occur when modifying native read-only raw files (.NEF), as the adjustments live strictly in the catalog. It specifically occurs when Lightroom attempts to write directly into a DNG container file over an SMB connection.

**Lightroom Classic Version Number:**

• Lightroom Classic v15.4.1 [ 202606101834-493069d4 ]

**OS Version Number:**

• macOS:

  • System Version: 26.5.1 (25F80)

  • Kernel Version: Darwin 25.5.0

  • SMB_VERSION SMB_3.1.1

• Windows 11 Pro

  • Version: 25H2

  • OS Build: 26200.8655

  • SMB Version: 3.1.1

**Storage Environment:**

• Synology NAS (4 x 8TB drives in RAID/SHR configuration)

• Model: DS920+

• DSM Version: DSM 7.3.2-86009 Update 3 (Latest Version)

• Connected via SMB protocol

  • Maximum SMB Version: SMB3

  • Minimum SMB Version: SMB2

  • SMB Range: SMB2, SMB2 and Large MTU, SMB3

  • Transport Encryption Mode: Client Defined

  • Server signing status: Disable SMB1 signing on

  • Enable Opportunistic Locking -> CHECKED

  • Enable SMB2 file leasing -> CHECKED

  • Enable SMB3 directory leasing -> UNCHECKED

  • Enable SMB durable handles (Cross-protocol file locking will be disabled) -> CHECKED

• Massive volume headroom: 4.2 TB photo volume with 2.8 TB of confirmed free space. (Rule out "disk full" or local permission issues).

**Step-by-step Reproduction Instructions:**

1. Store original raw files (.NEF) in a synchronized folder on an SMB-mounted Synology NAS share.

2. Select multiple .NEF files and perform a Photo Merge -> HDR to create a new Linear DNG.

3. Apply deep edits in the Develop Module (e.g., Transform settings, Super Resolution, or AI Generative Remove / distraction removal tools).

4. Return to the Library module, select the newly generated DNG, and go to Metadata > Update DNG Preview & Metadata.

**The Expected Result:**

Lightroom Classic successfully writes the updated XMP edit data and embeds a new full-size JPEG preview into the DNG container file over the SMB network path.

**The Actual Result:**

The write operation fails or gets interrupted mid-stream. The file header truncates. The image immediately gains a black exclamation mark icon (!). The image can no longer be edited in the Develop module ("unsupported or damaged"). Finder "Get Info" shows only the unedited original preview, proving the raw block behind the header was completely dropped or blocked during the write process.

**Troubleshooting Performed & Workaround:**

Recreating the exact same workflow with other random images yielded identical file corruption every single time on v15.4.1.

**The Definitive Solution:**

Completely rolled back the software to **Lightroom Classic v15.3.1** and restored a catalog backup from June 22nd. Under v15.3.1, the exact same workflow (HDR Merge -> Heavy AI Edits -> Update DNG Metadata) executes perfectly on the exact same network folder over SMB with zero errors or file corruption. This confirms a distinct network I/O code regression introduced in the 15.4 file-saving layer.