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I know that SD cards, like all things, have a limited lifespan and I know that they can handle a limited number of write cycles, which is usually what limits the lifespan. But what is an MTBF estimate for an SD card that is only wirtten to once and then used only for reads?

Here's why I'm asking. My car dealer is telling me that my navigation system is failing because the map SD card, which is set to read only, has gone bad after just a few years. I find this difficult to believe.

EMan L
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  • Thanks to both lgb and Tetsujin for comments and the quick replies. SD cards are cheap, but this one with the map data on it is not. I agree with everything that both of you have written. I'm going to go over to the dealer with my laptop and run a chkdsk on the card.

    I upgraded the maps a few years ago and I still have the original SD card that came with the car. Chkdsk produced no errors when I ran the utility on the card. If chkdsk shows errors on the current card, I will agree that the card is bad. Otherwise I'm going to ask that the old be popped in to see if the problem persists.

     – EMan L Dec 20 '18 at 17:52
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    If you want to detect bit rot, you should do SHA2 checksums on all the files, and verify each month (or etc) that they still match. – cybernard Dec 20 '18 at 20:07

2 Answers2

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It's quite likely - SD cards are delicate & fail easily.

I could find you the MTTF figures for your particular card, but it wouldn't really tell you anything except what it says - mean time to failure, the average life-expectancy of that specific model, not the life-span of any individual card.

I used to get through hundreds, if not thousands*, of SD cards for work. They would be write few, read many - so they'd have new data put on, which would then be inserted into a read-only machine for several months/years at a time, then cycled round again with new data.
Part of my job was to check the machines, repair/replace as necessary, throw away the failed SD cards & replace them - so I've seen one heck of a lot over 10 years of operation to form this opinion.

Some will rock on for years without a glitch. Some will start to fail in a few weeks. There's no way to tell which is going to do which.

The 'fix' is just to bin it & get another. It's pointless trying to fight them once they start to fail.
They're cheap, they're disposable.
They should never be used to store the only copy of any important information.

*I worked it out roughly - at any one time I would have just under 2,000 cards in the field, in constant use.

Tetsujin
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  • Yes, flash media is just like a car battery; on average it's expected that it will fail after X amount of time but it's impossible to know for any individual specimen. I've been driving around on the original battery in my car for 8 years now. It should have died a long time ago but it's still going strong. – Wes Sayeed Dec 20 '18 at 17:50
  • Tetsujin, you would be the first human to have ever published real life figures or estimates of SD card longevity on the web, if you had records or number of year estimates of greater detail. Some will rock on for years without a glitch. Some will start to fail in a few weeks. There's no way to tell which is going to do which. – bandybabboon Apr 27 '23 at 07:55
  • @WesSayeed - I just fell foul of that one… original battery lasted 12 years, its replacement I've just had changed again (under warranty) at 2 years :\ – Tetsujin Apr 27 '23 at 07:58
  • @bandybabboon - we never kept records. The only time we would pay it much attention was if we noticed a bad batch; then we'd send them back to the factory for testing. [We were buying direct from the actual manufacturers in China.] They were a tiny cost factor in a much larger operation & were always considered completely disposable. I still probably have a couple of hundred kicking around in ziplock bags. Too small these days to be much use, 1 & 2GB were common at the time. – Tetsujin Apr 27 '23 at 08:01
  • So, if install 10 reading kiosks in Nepal far away and difficult to maintain, with the SD card in read-only mode, will it on average last more than 3 years or less? more than 5 or less? what are the MTTF vaguely? – bandybabboon Apr 27 '23 at 08:01
  • @bandybabboon - I really don't know; we really didn't care. Later we swapped to better/faster structures built into the motherboards, which had much more resilience. [I'm afraid I don't know exactly the memory type these used.] – Tetsujin Apr 27 '23 at 08:04
  • I just found MTTF for NAND stated at 170 years... :) The SD has changed a bit since 2009, wear-levelling has become standard, that can read-write sectors that are accessed the most, it depends what algorythm is used, there are no wear-levelling benchmarks on line either!!! you can make an SD card fail in 1 hour if you use the same bit continually without levelling. – bandybabboon Apr 27 '23 at 08:08
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As explained here https://electronics.stackexchange.com/questions/162415/will-reading-serial-flash-memory-wear-it-out there is virtually no limit on how much reads a flash memory can support, BUT there is a phenomenon called read disturb, that can corrupt the data on the surrounding cells.

This happens with a very high number of read operations on the same block, and is usually taken care of by the controller of the flash memory. I am unsure if that process can happen at all if the SD is write protected.

Given that a SD card is a cheap replacement, I would say just buy one from a reputable brand and check if it fixes the problem. It's maybe BS from the dealer, but sure it can happen. As Tetsujin say, SD is not the most robust media ever, they can fail for no reason at all.

Igb
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    SD card write protect is only a software switch [which most systems respect but don't actually have to] so is actually not really relevant. – Tetsujin Dec 20 '18 at 16:53
  • @Tetsujin ok, thanks for the clarification. In that case, there is even less warranty that the SD card of OP recieved no writes. – Igb Dec 20 '18 at 16:54
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    If the machine is read only, then it would be 'certainty enough' though no guarantee. Presumably if the flash controller itself can handle moving data to try compensate for read disturb [which I hadn't heard about before, so thank you for that] then it may mitigate it - but practically, the darn things just fall over when they feel like it ;) – Tetsujin Dec 20 '18 at 16:56
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    In regards to "read disturb" and from reading the link above and Wikipedia link, it is only the data that gets corrupted, not the card itself. So, I would assume that reformatting the card and reloading data should work in this type of situation. – kojow7 Jun 09 '22 at 14:52
  • For an embedded system, you can use a second/third copy of the OS which is probably only a few GB, or less for an install like Alpine, and have an option which you want to boot to at start. – bandybabboon Apr 26 '23 at 11:37