What format does Arduino use for SD cards?
Arduino (including CosmicWatch-style data loggers) doesn’t define its own SD format — it relies on the SD card being formatted in a standard way that the Arduino SD library can read/write.
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💾 Correct SD card format for Arduino
✅ Required format
File system: FAT16 or FAT32
Best choice: FAT32 (for most cards >2 GB)
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📏 Card size rules (important)
SD size Recommended format
≤ 2 GB FAT16
4–32 GB FAT32
>32 GB (SDXC) Usually needs reformat to FAT32 (not exFAT)
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❌ What will NOT work
Arduino SD libraries (like in CosmicWatch firmware) typically fail with:
exFAT ❌ (very common on new cards)
NTFS ❌
APFS / Mac formats ❌
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🧪 Best practice for CosmicWatch v2
For muon logging stability:
Use 4 GB to 16 GB microSD
Format as:
FAT32
32 KB allocation unit (ideal, but default is usually fine)
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🧰 How to format properly
🪟 Windows
1. Insert SD card
2. Open File Explorer
3. Right-click SD card → Format
4. Set:
File system: FAT32
Allocation unit: Default (or 32 KB if available)
5. Click Start
If FAT32 is not shown (cards >32 GB):
use “guiformat” FAT32 formatter tool
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🍎 macOS
Use Disk Utility:
Select SD card
Erase
Format: MS-DOS (FAT)
Scheme: Master Boot Record (MBR)
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🐧 Linux
sudo mkfs.vfat -F 32 /dev/sdX1
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⚠️ CosmicWatch-specific tip
The SD logging code in CosmicWatch v2 firmware usually:
writes plain text or CSV files
expects short file names (8.3 format)
can be sensitive to slow or large SD cards
So avoid:
huge 128 GB cards
ultra-high-speed “A2 app performance” cards (sometimes unstable on AVR)
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🧠 Recommended setup (works reliably)
8 GB or 16 GB microSD
FAT32 format
Class 4–10 speed
freshly formatted before first use
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👍 If your SD logging is failing
Tell me:
what error shows in Serial Monitor (if any)
whether the card is detected at all
which CosmicWatch firmware you’re using
I can help you tune it so it logs muon events reliably without missed counts.
Window giving me options of exFAT or NFTS only and not FAT32.
That’s expected on Windows — it simply means the SD card is too large for Windows to offer FAT32 in the normal format dialog.
But for Arduino/CosmicWatch, you must not use exFAT or NTFS.
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⚠️ What you should NOT choose
❌ exFAT (won’t work with Arduino SD library)
❌ NTFS (also incompatible)
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✅ What you actually need
✔ FAT32 (required for CosmicWatch / Arduino SD logging)
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💡 Why you can’t see FAT32
Windows hides FAT32 for cards:
larger than 32 GB (SDXC cards)
So if your SD card is:
64 GB / 128 GB → Windows will only show exFAT or NTFS
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🛠️ Fix options
✅ OPTION 1 (BEST): Use a smaller SD card
For CosmicWatch v2, this is ideal:
4 GB
8 GB
16 GB
Then Windows will automatically allow FAT32 formatting.
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🛠️ OPTION 2: Force FAT32 on large card (recommended if you must use it)
Use a FAT32 formatter tool:
Windows tool:
“guiformat” (also called FAT32 Format tool)
Steps:
1. Download FAT32 formatter (guiformat)
2. Run as admin
3. Select SD card drive
4. Set allocation unit: 32 KB (default OK too)
5. Click Start
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🐧 OPTION 3 (advanced): Command line (Windows)
For 32 GB cards sometimes works:
format X: /FS:FAT32
(Replace X: with SD drive letter)
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🧠 Important CosmicWatch note
The CosmicWatch v2 firmware expects:
FAT16/FAT32 filesystem
simple file structure
no exFAT metadata overhead
So exFAT cards often cause:
“SD init failed”
silent logging failure
missing data files
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👍 Best practical recommendation
For your muon detector:
👉 Use a 8–16 GB microSD card formatted FAT32
That will give:
highest compatibility
least firmware issues
most reliable long-term logging