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The SD card must be FAT formatted. Here are the reasons why:

Prompt

Would you know if Linux file systems could be supported? Does Arduino just limit itself to FAT?

Response

Excellent question. This gets into architecture, not just formatting.

Short answer:

Yes — Arduino’s SD library is effectively limited to FAT (FAT16/FAT32, sometimes exFAT). No — Linux filesystems like ext4 cannot realistically be supported on an ESP32-class microcontroller.

Now let’s unpack why.

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1. What Arduino Is Actually Using

When you call:

SD.begin(...)

You are using:

• Arduino-ESP32 SD wrapper
• Which wraps FatFS
• Which implements FAT16/FAT32
• Sometimes exFAT depending on build configuration

FatFS is small, deterministic, and designed for microcontrollers.

It is not a Linux VFS stack.

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2. Why FAT Is Used in Embedded Systems

FAT has advantages:

• Extremely simple on-disk structure
• No journal
• No complex metadata trees
• Low RAM requirement
• Small code footprint
• Deterministic behavior
• Universally readable by Windows / macOS / Linux

That last point matters enormously for field devices like yours.

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3. Why ext4 (or other Linux filesystems) Are Not Practical

Let’s examine what ext4 requires:

• Journaling
• Block group descriptors
• Extents
• Multi-level inode structures
• Dynamic allocation bitmaps
• HTree directory indexing
• Crash recovery journal replay
• Kernel-level VFS layer
• Substantial RAM buffers

An ESP32-S3 has:

• ~512KB internal RAM
• 8MB PSRAM (external, slower)
• No MMU
• No kernel
• No block device abstraction layer like Linux

You would need to:

• Port the entire ext4 driver
• Provide block caching
• Provide journaling layer
• Handle power-loss atomicity

It would consume massive RAM and flash and be very fragile.

In practice: not realistic.

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4. What You Could Use Instead

Option A — FAT (what you're using)

Best choice for removable SD cards.

Option B — LittleFS (internal flash)

For internal storage (SPI flash partitions), many embedded systems use:

• LittleFS
• SPIFFS (older)
• Wear-leveling FS

These are:

• Designed for microcontrollers
• Power-loss resilient
• No journaling overhead
• Small RAM footprint

But they are not suitable for large SD cards.

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5. Could exFAT Be Used?

Possibly.

ESP32’s FatFS can be built with exFAT support (depending on SDK config).

exFAT supports:

• Large volumes
• Large files
• More modern structure

But still lightweight compared to ext4.

However, FAT32 is usually sufficient for logging systems like yours.

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6. Important Architectural Insight

Your SD card is not just storage — it is a removable exchange medium.

If you format it ext4:

• Windows cannot read it.
• macOS cannot read it.
• You lose plug-and-read convenience.
• Field debugging becomes painful.

For devices like:

• LoRa nodes
• Data loggers
• Field sensors

FAT is the industry standard specifically because it is portable.

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7. If You Truly Wanted Linux Filesystems

You would need:

• Embedded Linux
• A kernel
• ext4 driver
• Block device driver
• MMU
• Proper memory management

That means something like:

• BeagleBone
• Raspberry Pi
• Not ESP32

You crossed that boundary.

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8. So Is Arduino “limiting” you?

Not really.

The limitation is:

• Microcontroller architecture
• RAM footprint
• Deterministic real-time constraints

Arduino simply wraps FatFS because it is appropriate for this class of device.

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9. For Your Specific Project (microReticulum + T-Beam)

FAT32 is actually ideal because:

• You want easy card extraction
• You want to analyze logs on Linux workstation
• You want simple directory trees
• You do not need journaling
• You do not need Unix permissions

Your current design is correct for the platform.

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10. Advanced Thought

If you ever wanted:

• Transaction-safe logging
• Crash-resistant metadata
• Append-only log guarantees

You would implement that at the application layer, not by switching to ext4.

For example:

• Write logs with length-prefixed records
• Use atomic rename patterns
• fsync after writes
• Maintain journal file in userland

That’s how embedded systems solve it.

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If you’re curious, I can also explain:

• Why FAT corruption risk is often overstated on embedded devices
• Or how to implement power-fail-safe logging patterns on FAT

Both are relevant to your field nodes.