Yes — a two-buffer scheme is a good start, but it only **reduces exposure**. It does not make FAT-on-SD immune to mid-write interruption.

What I mean by “immune” is narrower and more mechanical:

> after a brownout, reset, card glitch, or socket bounce, the logger may lose only the record being written, but it does not corrupt earlier records or the directory structure.

Espressif’s own guidance points in this direction. Their file-system guidance says LittleFS is designed for microcontrollers, has power-failure resilience, and does wear levelling; their FAQ also says FatFs does not support write transactions and accidental power loss can damage partitions, so mitigation has to happen at the application level. ([Espressif Systems][1])

For your use case, I would separate the options into three tiers.

## Best practical approach for this project

### Append-only fixed-record journal in internal flash

Instead of writing CSV lines to FAT, write fixed-size binary records into a reserved flash partition as a ring buffer or append-only log.

Each record would contain something like:

* magic number
* record version
* sequence number
* monotonic ms since boot/run
* payload length
* payload
* CRC32

And the write rule would be:

1. prepare complete record in RAM
2. write one contiguous record to next flash slot
3. verify CRC on readback if desired
4. advance write pointer only after success

On reboot, the parser scans forward until:

* bad magic
* bad CRC
* impossible length
* erased flash pattern

Then it stops. Everything before that point is valid.

That is the model I would call “near-immune.” A power cut may destroy the last record, but not the whole log.

## Why this is stronger than FAT

FAT has shared metadata: directory entries, FAT chains, timestamps, free-space accounting. A torn metadata write can damage the filesystem namespace, which is close to what your `fsck.vfat` output showed. FatFs itself is not transactional under accidental power loss. ([Espressif Systems][2])

With an append-only journal:

* no directory updates during logging
* no long filename metadata
* no cluster allocation churn
* recovery is linear and deterministic

## Good filesystem-based alternative

### LittleFS on internal flash

If you still want “files,” LittleFS is the strongest ESP-IDF choice for this class of device. Espressif describes it as having a good level of power-failure resilience and dynamic wear levelling, and recommends it as a general choice for embedded applications. ([Espressif Systems][1])

I would use LittleFS only for:

* configuration
* exported snapshots
* occasional summary logs

I would still prefer a binary journal for the high-frequency raw stream.

## What I would not trust for raw high-rate logging

### FAT on SD as the primary live log

Even with double buffering, it still depends on:

* socket contact
* card controller behavior
* shared SPI stability
* directory/FAT updates eventually occurring

So buffering improves durability, but the architecture remains fragile. Espressif’s FAQ is explicit that FatFs power-loss damage must be handled at application level. ([Espressif Systems][2])

# Concrete design I would recommend

## Partition layout

Reserve an internal flash partition, for example:

* `config` — NVS or LittleFS
* `rawlog_a` — append journal
* `rawlog_b` — optional mirror / rollover / generation swap
* `export` — optional LittleFS area for human-readable dump files

## Record format

Something like:

```c
struct log_record_hdr {
    uint32_t magic;        // 0x54424C47, for example
    uint16_t version;
    uint16_t type;         // sample, satellite, event, marker
    uint32_t seq;
    uint32_t ms_since_run_start;
    uint16_t payload_len;
    uint16_t hdr_crc;
    uint32_t payload_crc;
};
```

Followed by compact binary payload.

That gives you:

* forward compatibility
* corruption detection
* clean stop-point on reboot

## Runtime behavior

* Sample task writes into RAM buffer A
* When A fills, swap to B
* Background flush task writes A as a batch of complete records
* Never rewrite old records in place
* Never “update header counts” during the run
* At end of run, optionally write a single end marker record

## Export behavior

After the run, or on explicit command:

* convert binary journal to CSV
* stream out over USB serial, Wi-Fi, or copy to SD if present
* if SD fails, raw evidence still survives in flash

That is the key architectural shift:

> log safely first, prettify later

# If you want redundancy

You mentioned Codex designed a two-buffer system. I would reinterpret “two-buffer” at the storage layer too:

### Option A — dual-generation partitions

Write runs alternately to `rawlog_a` and `rawlog_b`, each with a generation counter in a superblock. On boot, choose the highest valid generation with good CRC. This is similar in spirit to Espressif’s own “two copies” suggestion for handling FatFs power-loss exposure. ([Espressif Systems][2])

### Option B — mirrored commit blocks

For metadata like run header, end marker, and export manifest, store two copies with generation numbers and CRCs. Never rely on a single rewritten sector.

# Wear concern

The obvious objection is flash wear. That is real, but manageable if:

* records are compact
* you use a ring buffer
* you erase in blocks and append sequentially
* raw logs are treated as field-session data, not archival forever

Espressif’s storage guidance explicitly highlights wear levelling for embedded filesystems like LittleFS and SPIFFS. ([Espressif Systems][1])

# My ranking for your T-Beam logger

1. **Best:** raw append-only binary journal in internal flash, then export later
2. **Good:** LittleFS on internal flash for modest-rate logging
3. **Acceptable only as secondary/export:** FAT on SD
4. **Worst:** live CSV append to FAT on SD with frequent closes

# My blunt answer

A logging approach that is “immune” in the engineering sense would be:

> an append-only, CRC-protected, fixed-record journal in internal flash, with no filesystem metadata churn during capture.

That will not make physics go away, but it changes failure from:

* “filesystem damaged, maybe whole log compromised”

to:

* “last partial record lost; all prior records intact.”

That is a much better failure mode.

If you want, I’ll sketch the exact partition table and a Perl-friendly binary record schema so your post-run converter can turn the journal into the same CSV structure you already import.

[1]: https://docs.espressif.com/projects/esp-idf/en/stable/esp32/api-guides/file-system-considerations.html?utm_source=chatgpt.com "File System Considerations - ESP32 - — ESP-IDF ..."
[2]: https://docs.espressif.com/projects/esp-faq/en/latest/esp-faq-en-master.pdf?utm_source=chatgpt.com "ESP-FAQ Handbook - Technical Documents"
