Which microSD a hotspot needs: speed class, capacity and how not to get a fake

The memory card is the most underrated link in a Raspberry Pi DMR hotspot. Everyone fine-tunes the antenna and frequency calibration, then trips over a penny microSD: the hotspot lives a week, then “won't boot,” the logs are garbage, and you have to re-flash the image. The card holds the entire system — Linux, the MMDVM software, configs, the operating logs — and it's written to constantly, around the clock. A cheap card or, worse, a counterfeit one dies quickly under 24/7 use and drags a corrupted filesystem down with it. Let's lay it out in plain terms: what capacity and speed class a hotspot really needs, what all those A1/A2/U1/High Endurance markings mean, and how not to buy a “512 GB for $3” card that's actually empty inside.

How many gigabytes you really need

A hotspot doesn't need much capacity — it's not a media player. The ready-made RadioStar image, together with the system, the MMDVM software and the panel, takes up just a few gigabytes, and the operating logs are text that doesn't add up to much. In practice:

• 8 GB — the working minimum, the image fits with room to spare.
• 16 GB — the sweet spot: a bit more room for logs and updates, while the price barely differs from 8 GB.
• 32 GB and up — overkill. There's no point paying for capacity you'll never use.

A market paradox: buying an honest small-capacity card is hard these days — the brand-name lines start at 32 GB. That's fine: get a genuine 32 GB if you can't find 8–16 — the markup is trivial, whereas for a “huge capacity at a bargain price” you're most likely paying for a fake (more on that below).

Speed class: what the symbols on the card mean

A whole set of markings is printed on a microSD's body. Not all of them matter for a hotspot, but it helps to understand them:

The key idea almost everyone misses: what matters for the system is not sequential speed, but random access — IOPS. Linux on a hotspot constantly pokes at lots of small files rather than writing one big stream. Random access is exactly what the A1 and A2 classes (Application Performance Class) cover, not the flashy “90 MB/s” on the front. A card can boast a high sequential speed and still bog down the system if its IOPS are low.

High Endurance: why it matters for a hotspot

A hotspot is a device that writes to the card constantly, for years, without ever being switched off. Every write wears out the flash memory cells, and their lifespan is finite. An ordinary card under continuous writing lasts 1–3 years, after which it starts to fail. The High Endurance (SanDisk) and Pro Endurance (Samsung) series were designed from the start for dashcams and surveillance cameras — that is, for exactly the “writing around the clock” scenario. They have many times more allowable write cycles.

For a hotspot this is a perfect match for the task: the main wear comes precisely from the operating logs the system writes continuously. If you have the choice and a small markup doesn't scare you — get an endurance series. It genuinely extends the card's life.

Brand or no-name: why a cheap card costs more

The rule is simple: buy a genuine card — SanDisk, Samsung or Kingston. Not out of snobbery, but because no-names and fakes don't pass quality control and behave unpredictably as the carrier of an entire OS. Here's what a bad card does in a hotspot:

• Filesystem corruption — superblock errors, bad sectors, and the system stops booting.
• “Mysterious” failures — the hotspot randomly freezes, reboots, loses its config. You blame the software, but the card is at fault.
• “Won't boot” — the classic ending: the image was flashed, everything worked, and a month later the Pi won't start. The card quietly degraded.

A cheap card costs more than a genuine one: you waste time reinstalling the image, chase floating glitches, and in the end go buy a proper one anyway. Better to buy the right one once, up front.

Fakes: “512 GB for pennies” is a myth

A separate headache is counterfeit cards. Marketplaces and cheap shops are full of “cards” with huge capacity for laughable money. The scheme is always the same: the card's controller is reflashed to report a false capacity to the system. The computer sees “512 GB,” but there are only 8 or 16 GB of real memory inside. When writing exceeds the real capacity, data starts being written “in circles,” overwriting what was already there — and everything collapses without warning.

Where the risk of getting burned is especially high: anonymous sellers on AliExpress, Wish, Temu and similar platforms. If the price looks too good to be true — it's a fake.

How to buy safely:

• Buy from a trusted seller — a major retailer, the brand's official store, or an item “sold and shipped” by the marketplace itself rather than a third-party seller.
• Check the card's model against the manufacturer's website — fakes often list nonexistent part numbers.
• Look closely at the printing: smudged, crooked or faint type on the card and packaging, “off” logos — a sure sign of a fake.

How to test a card: H2testw and F3

Two free tools check a card's real capacity and integrity by filling it with data and reading it back with verification. If the card lies about its size or is failing — the test catches it.

• H2testw — for Windows. Insert the card, select it, choose to check all available space, set the mode to Write + Verify and run it. A green “Test finished without errors” result means the card is honest. Red messages like “X bytes corrupted,” an early stop, or I/O errors mean a fake or a defective unit.
• F3 (F3 Flash) — for Linux and macOS, the same principle. It consists of two utilities: f3write packs the free space with 1 GB files, and f3read reads them back and verifies them.

On Linux (including straight on another Pi) it looks like this:

# install F3 (Debian/Ubuntu/Raspbian)
sudo apt install f3

# the card is mounted, e.g. at /media/usb
f3write /media/usb        # fill the free space
f3read  /media/usb        # read back and verify

# quick check of the real capacity without a full write:
sudo f3probe --destructive --time-ops /dev/sdX   # WARNING: erases the card

If f3read reports corrupted sectors, or the real capacity turns out to be smaller than advertised — scrap the card, don't flash an image onto it.

Connection to the RadioStar image

Everything above directly affects whether your hotspot survives the image flash. The RadioStar image is written to this very microSD, and the system runs off it from then on. So the order is:

1. Buy a genuine 8–16 GB card (A1/A2, Endurance if possible).
2. Test it with H2testw/F3 — confirm the capacity is real and there are no errors.
3. Only then flash the image of RadioStar and build the hotspot.

This extra testing step seems like a chore right up until the first time a hotspot “mysteriously” crashes once a week. With a proper, tested card of this class the problem simply doesn't arise — and the whole “the image got wiped, I don't get it” saga passes you by.