Tahoe Time Machine Failure on Samsung T7

ISSUE

Every good storage failure starts innocently.

Plug in a brand-new Samsung T7.
Select it in Time Machine.
Click “Use Disk.”
Allow macOS to format the drive automatically.
Let the backup run. It may even complete once — maybe even a few more times.
Then it fails with a generic error:
“An error occurred while preparing the backup.”

Time Machine consistently failed during the “Preparing backup” phase on macOS 26.3 (Tahoe).

We checked the usual villains:

Everything looked correct.

Observed Symptoms

Backup stalled at:

BackupPhase = PreparingSourceVolumes
Percent = -1

Error message:

“An error occurred while preparing the backup.”

I then ran:

log show --last 10m --predicate 'process == "backupd"' --style syslog | tail -40

backupd log showed:

[TMSnapshot] fs_snapshot_list failed: Operation not supported

• Internal APFS snapshot creation (tmutil snapshot) worked instantly.
• Destination disk (Samsung T7 4TB) was APFS formatted and showed:

Role: Backup

Despite correct filesystem type and role assignment, Time Machine refused to proceed.

DISCOVERY

diskutil list disk2

Inspection of the physical disk layout revealed:

/dev/disk2 (external, physical):
   #:                        TYPE NAME                SIZE       IDENTIFIER
   0:      GUID_partition_scheme                    *4.0 TB      disk2
   1:      Microsoft Reserved                        16.8 MB     disk2s1
   2:      Apple_APFS Container disk3                4.0 TB      disk2s2

There it was. A tiny 16.8 MB Microsoft Reserved partition the Microsoft Windows fingerprint. Samsung ships T7 drives ‘cross‑platform’. They arrive factory‑formatted as exFAT for Windows compatibility.

When Time Machine reformats an exFAT volume to APFS, it:

• Replaces the selected partition
• Does NOT wipe the entire physical disk
• Leaves unrelated GPT entries intact

So we ended up with:

• APFS container sitting on top of
• A hybrid GPT layout
• Containing a Windows MSR artifact

This wasn’t the first time I’d seen odd behavior from the same model drive under Tahoe. On a separate occasion, file transfers from a Samsung T7 — already formatted as APFS — would intermittently fail. At the time, I attributed it to a flaky cable, which is statistically the usual suspect. But in hindsight, that drive had also been reformatted at the volume level rather than the physical device level. Given what we now understand about Tahoe’s stricter interaction with APFS containers and underlying GPT layout, it’s highly likely the earlier file transfer failures had the same root cause: a hybrid partition map left intact beneath an otherwise valid APFS container.

Key Findings

• The Samsung T7 shipped with a Microsoft Reserved (MSR) partition, typical of factory cross-platform formatting.
• When Time Machine reformatted the disk from exFAT to APFS, it only erased the exFAT partition.
• The Microsoft Reserved partition remained in the GPT layout.
• The APFS container was therefore sitting on a hybrid GPT structure.

This caused:

• fs_snapshot_list to return “Operation not supported”
• Time Machine to fail during destination snapshot validation
• Backup to halt at “Preparing”

Once the entire physical device was erased via:

diskutil eraseDisk APFS TimeMachine GPT disk2

The (correct) layout became:

/dev/disk2 (external, physical):
   #:                        TYPE NAME                SIZE       IDENTIFIER
   0:      GUID_partition_scheme                    *4.0 TB      disk2
   1:      EFI EFI                                   209.7 MB    disk2s1
   2:      Apple_APFS Container disk3                4.0 TB      disk2s2

No Microsoft Reserved partition remained.

Backup then proceeded normally.

CONCLUSION

The failure was caused by a residual Microsoft Reserved (MSR) partition left from factory formatting.

Time Machine reformats selected volumes, but it does not necessarily wipe the entire physical disk. Modern macOS versions (Big Sur → Sonoma → Tahoe) expect clean APFS snapshot support at the container level.

With a hybrid GPT layout present, destination snapshot enumeration failed:

fs_snapshot_list failed: Operation not supported

After fully erasing the physical device and rebuilding a clean GUID + APFS structure, Time Machine operated normally.

FINAL DETERMINATION

This was not:

• A permissions issue
• A corrupted snapshot on the internal disk
• An APFS role misconfiguration
• A hardware failure

It was a factory Windows GPT artifact left beneath an otherwise valid APFS container.

Erase the physical disk — not just the volume.

How Time Machine Actually Broke

Modern Time Machine is not a simple file copier; it is tightly integrated with APFS and depends on snapshot semantics. When a backup begins, it interrogates the destination container, attempting to enumerate and validate snapshot capability through APIs such as fs_snapshot_list(). If the APFS container sits on a clean Apple GUID partition map, enumeration succeeds and the backup proceeds. But storage is layered — an APFS volume inside a container, inside a physical store, inside a GPT map — and if that lowest layer contains foreign artifacts like non‑Apple GPT entries, Microsoft Reserved slices, or hybrid remnants, snapshot enumeration can fail.

The kernel responds:

Operation not supported

Because the container metadata and physical layout do not align with the snapshot support configuration macOS expects.

This is subtle. It is not documented as:

“Time Machine fails if MSR partition exists.”

The partition map was dirty. APFS is layered. Snapshot enumeration travels down the stack. If the lower layer smells like Windows, Tahoe refuses to play.