Speed of SD Cards vs USB Pendrives

Good question — the speed difference between SD cards and USB flash drives (pendrives) depends on several factors, but generally:

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🧭 1. Interface Speed Matters Most

Type	Typical Interface	Theoretical Max Speed
Standard SD / microSD	SD (UHS-I)	~104 MB/s
High-end SD (UHS-II/III)	SD (UHS-II/III)	~312 MB/s (UHS-II), ~624 MB/s (UHS-III)
USB 2.0 Pendrive	USB 2.0	~35 MB/s (practical ~20 MB/s)
USB 3.0 Pendrive	USB 3.0 (5 Gbps)	~625 MB/s (practical 100–300 MB/s)
USB 3.2 / USB-C Pendrive	USB 3.2 (10 Gbps)	Up to 1 GB/s (practical 300–800 MB/s)

Summary:

• Cheap USB 2.0 pendrives are often slower than even modest SD cards.
• Good USB 3.0 drives can outperform most SD cards, except high-end UHS-II/III cards.
• UHS-II/III SD cards require a compatible reader to reach their top speeds.

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📸 2. Read vs Write Speeds

• SD cards often have fast read but slower write speeds, especially in cheaper models.
  • Example: a 95 MB/s card might only write at 30–60 MB/s.
• Many USB 3.0 drives have balanced read and write speeds, though cheap ones may still write slowly.

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⚡ 3. Real-World Performance

• SD cards are optimized for steady sequential writes (good for cameras, SDR data logging, etc.).
• USB drives can handle larger random writes better in many cases, especially higher-end ones.

For radio astronomy data logging or SDR capture (like with SDRplay RSPduo), a high-quality UHS-I or UHS-II SD card is often fast enough, but:

• if your system has USB 3.0, a good pendrive or SSD can give more headroom for sustained write speeds.

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🧪 Example (typical real-world numbers):

Device Example	Read Speed	Write Speed
Cheap USB 2.0 stick	20 MB/s	5–10 MB/s
Mid-range USB 3.0 stick	100 MB/s	40–80 MB/s
Fast USB 3.0 stick	200 MB/s	100+ MB/s
UHS-I SD card (Class 10 / V30)	80–95 MB/s	30–60 MB/s
UHS-II SD card (V60 or V90)	250+ MB/s	100–250 MB/s

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🧠 Key Tips

• Check the bus type and speed class (e.g., V30, V60, V90 for SD).
• For pendrives, make sure it’s USB 3.0 or higher.
• Sustained write speed is more important than peak if you’re recording data streams.
• If reliability matters, consider a small SSD over either SD or USB stick.

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Would you like me to compare SD vs USB specifically for continuous SDR data logging (e.g., for your interferometry work)? That case has some unique considerations.

Excellent — for continuous SDR data logging (such as interferometry at 1420 MHz with SDRplay RSPduo), sustained write throughput and reliability are much more important than the headline “maximum speed” figures printed on packaging.
Here’s a focused comparison of SD cards vs USB pendrives for that specific use case:

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📡 Typical SDR Data Rate

For interferometry, say:

• Sample rate: 31 000 samples / s (per channel)
• Data type: complex64 (8 bytes/sample)
• Channels: 2

[
31,000 \times 8 \times 2 \approx 496,000\ \text{bytes/s} \approx 0.5\ \text{MB/s}
]

Even with overhead, under 2 MB/s sustained write speed is usually sufficient for baseband logging.
BUT… you want headroom (×5 to ×10) to avoid corruption from buffer overruns, filesystem latency spikes, or OS tasks.

👉 Recommended sustained write speed: at least 10 MB/s.

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🧱 SD Cards for SDR Logging

Feature	Typical Performance	Pros	Cons
UHS-I V30 (common)	30–60 MB/s write	Cheap, compact, low power, good for sequential writes	Speed degrades with age; many fake cards; controller wears unevenly
UHS-II V60 / V90 (high-end)	100–250 MB/s write	Excellent sustained rate, low jitter	Needs compatible reader; expensive
microSD (UHS-I)	20–50 MB/s write	Easy to use with SBCs (e.g. Raspberry Pi 4 Model B)	Small form factor → easier to damage; lower endurance

✅ Strengths:

• Designed for continuous streaming (cameras, dashcams).
• Works well with SBCs and embedded systems.
• Easy to integrate.

⚠️ Cautions:

• Avoid cheap “Class 10” with no V-rating — many drop to <10 MB/s after a few minutes.
• Buy from reputable brands (e.g. SanDisk, Samsung, Lexar).
• Use f3write or similar tools to test actual sustained performance before deployment.

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🖴 USB Pendrives for SDR Logging

Feature	Typical Performance	Pros	Cons
USB 2.0 stick	5–20 MB/s write	Very cheap	Often too slow; bottlenecks easily
USB 3.0 stick (mid-range)	40–80 MB/s write	Good headroom	Variable quality; performance can degrade under sustained writes
USB 3.0 “pro” models	100–300 MB/s write	Excellent headroom	Still lower endurance than SSD

✅ Strengths:

• More consistent write speeds on good models.
• Simple plug-and-play for desktops/laptops.

⚠️ Cautions:

• Many cheap USB 3.0 sticks buffer to fast SLC cache and then plummet to 5–10 MB/s after a few GB.
• Controller throttling and overheating are common in long runs.
• More prone to write latency spikes than good SD cards or SSDs.

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🆚 Summary: For Continuous SDR Logging

Metric	SD Card (UHS-I/II)	USB Pendrive (USB 3.0)
Sustained write	✅ Generally stable (V30 or better)	⚠️ Varies widely
Endurance	✅ Good (designed for streaming)	⚠️ Often lower
Latency spikes	Low	Moderate
Price per GB	Lower	Slightly higher
Long runs (1–4 h)	Reliable with quality cards	Risk of thermal throttling
SBC compatibility	Excellent	Sometimes needs powered hub

👉 Winner for long interferometry sessions:
Good UHS-I (V30+) or UHS-II SD cards, assuming your hardware supports it.

👉 Best overall option if budget allows:
A small external SSD over USB 3.0 — these give rock-solid sustained performance (300 MB/s+), no cache drop-off, and much higher endurance.

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🧰 Practical Tips

• Use rsync or dd tests to verify actual sustained write speed before logging.
• Mount SD/USB with noatime to reduce write amplification.
• Format in exFAT or ext4 for better performance on large files.
• Log to a dedicated partition or device to avoid OS contention.
• Monitor with iostat or dstat during a dry run.

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✅ Recommendation for your interferometry setup

• Use a UHS-I V30 (or better) SD card for reliability.
• Keep a USB 3.0 SSD as a backup or for longer runs.
• Avoid cheap USB sticks unless fully tested for sustained writes.

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