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I have some SD cards that I use. Is there a limit to how many times I can write over them? If I leave information on a card, is there anything that can cause it to lose this information eventually (other than a magnetic field)? What's the preferred storage conditions for these cards?

Wuffers
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Yuval
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  • There is contact wear concerns also...http://superuser.com/questions/405942/how-much-does-it-wear-an-sd-card-to-be-frequently-removed-reinserted – Moab Mar 28 '12 at 22:24
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    Be aware of static electricity discharges too; they might destroy your card... and it's easy to get static electricity from all the rubbing on the plastic clothing - unlike the lava thing or the train. –  Oct 26 '12 at 09:33

6 Answers6

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Flash memory indeed has limited write cycles. However, by now it is unlikely that you'll encounter this within the normal lifetime of such a card. Usually this is in the order of 100,000 write cycles today and SD cards include circuitry to manage wear-leveling, that is, spread out writes over the storage media evenly to avoid "hot spots"—pages that are written too frequently and therefore failing early.

Information stored on the card is safe even in magnetic fields because the information is not stored magnetically (contrary to hard drives or floppy disks).

As for storage conditions ... you shouldn't store them in mud, water, lava or other harmful conditions. You probably also shouldn't put them on railways and let trains drive over them. Apart from that, not paying particular attention where I store my cards I haven't had any adverse effects so far. In practice I'd think whatever doesn't physically damage the card won't harm the data on it.

Joey
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    @Johannes, How about booting from such memory? http://superuser.com/questions/681/booting-linux-off-usb-pendrives – nik Aug 03 '09 at 01:14
  • nik: Never tried myself but works. We've installed a traffic shaper here in the dormitory which runs off a CF card. However, we don't expect much writes there, so that may be a consideration to make before doing so. – Joey Aug 03 '09 at 07:54
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    Seriously though don't let static electricity touch the gold contacts. The data might be OK but the microcontroller in the card would be fried. – LawrenceC Nov 20 '11 at 02:52
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    SD cards do *not* have 100,000 write cycles, at least not for any reasonably priced ones. SLC flash sometimes does, not MLC. See [actual test data](http://forum.embeddedarm.com/showthread.php?3-SD-card-endurance-test). Or [here](http://www.storagesearch.com/ssdmyths-endurance.html). – derobert Jul 30 '13 at 15:38
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    Anecdotal data: I've had two micro SD cards fail on me in the past two weeks (a 16GB and a 2GB). These cards were more or less permanently mounted in a device - so no insertion/removal wear. As far as I know they didn't have unusual write activity; these were just in phones for whatever Android phones do with them. Suddenly the phone says that there's no SD card, and nothing will read or write to them - not phones, not Windows, not Linux (in a variety of machines). I'm not sure what this tells anybody other than it doesn't necessarily take a lava flow to render an SD card non-functional. – Michael Burr Dec 28 '13 at 09:10
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    Well, to be fair, the question was more about environmental and storage conditions. Android, depending on the apps, can be fairly write-happy. And I seem to have misread the write cycle reliability; at least Wikipedia tells a different tale with just 3k–5k cycles. – Joey Dec 28 '13 at 12:49
  • One thing is how long the manufacturer says they last. We all know flash memory of all sorts (SSD, SDCard, pen drive) they all get slower as you use them. At some point they become so slow on the I/O process that they will be unpractical to use. But manufacturers will say. "They still can do I/Os". So regardless of the spanlife, they all have their I/O speed decreased as you use them. – Azevedo Jul 09 '20 at 14:09
  • Since this was written rated life cycles have dropped dramatically. No cards these days are SLC. Cheaper ones are TLC and expensive ones are MLC at best. Those have endurance cycles in the hundreds, and in the thousands, respectively - NOT 100,000!! The good news is that capacities are also increasing and this helps spread out these cycles over a larger area, TBW wise, even with the poorest dynamic wear leveling usually employed in these cards. You can get "endurance" rated cards for dashcams etc but there's still no guarantee these use MLC and not TLC, but they at least should do more cycles. – thomasrutter Oct 10 '20 at 00:14
  • ..note: slowing down as it gets older or more filled is more typical of SSDs with more complex static wear leveling and higher performance, and it can be mitigated with TRIM optimisation and (due to static wear leveling) with some idle time to recover. Slowing down over time is not so much a thing in SD/microSD cards. There may be exceptions. The usually failure mode of an SD/microSD will be that one day it works, and one day it doesn't, either all or part has gone read-only or all of part is completely unreadable. – thomasrutter Oct 10 '20 at 00:20
  • Further to my comments from a few years ago, despite the falling number of rated write cycles in modern cards, the massive capacities of modern cards are making write cycles once against almost irrelevant. A 256GB card can be written to at full speed continuously for years before being able to reach its rated write cycles. The main driver of this is that capacities are expanding much faster than write speeds, so actually writing to every bit on the card only once takes hours – thomasrutter Feb 07 '23 at 06:25
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Never trust FLASH memory of any kind for long term storage. My experience with FLASH is integrity begins to falter in as little as 5 years. The voltages stored in the FLASH memory cells dissipate and can be misinterpreted after a while. High temperatures will accelerate the dissipation and shorten storage even less than 5 years.

High density FLASH where one cell can represent 2 bits has greatest failure rate.. most common in use as it is the cheapest to manufacture (double capacity). Many FLASH micro-controller applications are failing after 5-10 years because of FLASH memory corruption. Re-FLASHing the firmware restores the chip for another 5-10 years etc. So you must refresh FLASH memory data periodically to ensure continued integrity. Same would apply if you wanted to use the SD card as long term storage.

The FLASH memory chip itself is extremely rugged and can offer hundreds, if not thousands of years of service; as long as maximum write cycle lifespan has not been exceeded. FLASH memory cells are like millions of individual microscopic batteries that are charged at different levels. As you know, any battery new in the pack sitting on a shelf for years will eventually discharge. Same applies to FLASH memory cells, they require "recharging" periodically to maintain proper charges which represent data bits. So a FLASH memory card put into a safety deposit box for 25 years... guaranteed you will have corrupt data when you try to use it. I have seen FLASH memory with 20% corruption after 10 years of sitting in storage.

FLASH thumb drives are great for transferring data from one computer to another etc, but NEVER to be used for long term archival storage. Same goes for the conventional CD ROM. After 5-10 years, the contrast dye will fail causing read errors.

The best medium for long term storage are ARCHIVAL GOLD CD-R or similar brand. The storage medium on the disk is a thin layer of 24K gold so it will not oxidize. Data retention on this type of CD is expected to be intact even after 300 years. They are expensive compared to conventional CD-R.

Gavin Curtis
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  • @zingle-dingle This phenomenon has a name: `bit rot`. There are many discussion around, e.g. https://news.ycombinator.com/item?id=9759836; https://getprostorage.com/blog/bit-rot-stop-destroying-your-data/ – Ben Dec 15 '18 at 13:48
  • A crucial question would be: do microsd card manufacturers include background data scrubbing in their chips to mitigate bit rot while being powered? periodically reading the whole device actively would be a good idea. That should trigger internal error correction, which should rewrite bad cells. otherwise, copying the whole dataset once a year is the safest option. – korkman Jan 18 '19 at 13:07
  • 5 years? Hah! My HDDs don't even last that long. – Tanath Feb 26 '19 at 04:24
  • @shellter: just plug them in any (turned on) device, such as a PC. Their microcontroller will automatically refresh the data charge – MestreLion Nov 21 '21 at 14:56
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Durability

In my experience, memory cards are quite durable, though occasionally finicky when it comes to formats. I recently ran a cellphone through the washing machine (it was so dirty), and the 2 GB microSD card works just fine in my new phone (I eventually got the washed phone working too, but it was a good excuse to upgrade).

Rob Galbraith, who maintains an amazing website on CompactFlash and Secure Digital cards, says

Individual flash memory cells have a limited lifespan. That's the bad news. The good news is that their lifespan is usually measured in the many, many thousands of erase/write cycles, and that card controllers use an algorithm that balances the wear across the entire card's cells. CompactFlash and SD/SDHC cards are designed to automatically and transparently map out memory cells that go bad, or in some cases when they reach a predefined limit.

Write cycles are important, but MTBF (mean time between failures) is often 1M-2M hours or more, factoring in advances such as wear leveling, bad-block marking and management, etc.

Tips

  • Do not defrag a memory card. This consumes write/erase cycles and shortens the MTBF.
  • Use FAT32 instead of a journaling file system (like NTFS), which will write more often.
  • SD cards are rated to hold data at something like 10 years sitting idle. I recall reading (not sure where) about re-energizing cards by occasionally inserting into a reader.

Anecdotes

The 2004 BBC article Digital memories survive extremes covers an interesting study by Digital Camera Shopper on the durability of memory cards.

The memory cards in most cameras are virtually indestructible, found Digital Camera Shopper magazine. Five memory card formats survived being boiled, trampled, washed and dunked in coffee or cola.

In 2004, there was an incident (covered happily in a SanDisk press release at the time) where a photographer's compact flash card survived a bridge explosion where the camera gear was set up so close to the blast that it was destroyed, but the CompactFlash card survived. Other incidents like plane crashes are hyped by SanDisk so much that, admittedly, I get nervous using other brands. That said, it's not always easy to get data from a damaged card. An atmospheric research balloon crashed in the Pacific Ocean and was recovered. One SD card was read easily but another required intervention from SanDisk, but it was eventually read.

Bill Biggart's photos from 9/11 survived the collapse of the second tower on a CompactFlash microdrive card.

Recovery

If you suspect a card may be getting flakey, or if you run into trouble reading a card, immediately create a backup of everything on the card. There are low-level recovery tools like TestDisk and PhotoRec that come in handy for this.

Peter Mortensen
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b w
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    That's durability of the physical flash chip, not the data. Flash memory suffer from `bit rot`, https://en.wikipedia.org/wiki/Data_degradation, this happends more often than you could notice, if your sd card is only for media file. But it's in most likely intolerable for critical code e.g. system bootloader etc. – Ben Dec 15 '18 at 13:11
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I've used several brands of SD cards in raspberry pi computers, and they usually start seeing memory corruption after a continuous uptime of anywhere between 1 to 3 months, larger SD cards seem to last longer, smaller SD cards wear out in just a couple of weeks.

Owl
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  • What the heck are you doing with your PIs?? I've got one that has been taking 120 pictures a day for the past five months, no problem with SD card so far. Also a small web server with uptime of almost 3 months, getting at least 50 hits a day, also no problems with SD. The camera has a 16G card and the server has an 8G which was previously used for over a year in my phone. I am sure they will get corrupt eventually, but "a couple of weeks" - you must be doing some insane reading/writing. – YemSalat Jun 16 '15 at 12:37
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    I have been using a raspi for a sprinkler controller, and just now, at the 3 year mark, the card has gone bad. Thousands of unrecoverable fsck errors. Perhaps the raspian just logs too much for the flash to manage? – voidref Jul 14 '15 at 00:32
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    voidref, this is a bit off topic, but that's what happened to me too, twice. After that i turned off swapping in raspbian, and also mounted the root filesystems as read only and then use a ram disk to write temp files. That seems to have worked for me. I think it happened so often for me because i didn't use high quality SD cards and used smaller capacities, so the same blocks were getting rewritten loads. Eventually a crucial file gets clobbered / corrupted and rpi kernel panics. – Owl Apr 03 '16 at 23:23
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    You need to add `noatime` to the mount options of all microsd partitions, especially on read-heavy operations. Even when not running a microsd, but a "real" ssd or hdd, noatime will increase your performance. – user1933738 Oct 04 '16 at 22:54
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    There's also `nodiratime`, accumulated `commit` also worth trying. – Ben Dec 15 '18 at 12:59
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    raspberry pis are somewhat special in their sd card corruption. They don't have a rechargable battery nor many electronic parts smoothing power variations, plus their power supplies are cheap. AFAIK that is one main source of corruption in this case, and doesn't apply to other uses of sd cards. – korkman Jan 18 '19 at 13:16
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I don't know if this will help, but...

We use µSD cards for the entire filesystem on an embedded device, so they see reads and writes for logging purposes as well as swap. It is a journalled filesystem (previous teams' decision) and I have seen a handful of failures in a population of say 200 devices, with some brands having more failures than others. Some are complete catastrophic failures, I can't read nor re-partition and re-format the card and some are simply filesystem corruption and a re-partition and re-write has them working again. We don't trust those to be sent into the field however.

They have only been in the field for a maximum of 3 years. Thank goodness the real information has already been sent to a database and stored.

Patrick Buick
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    I am facing a similar rollout of a Linux product with micro SD cards as the RFS and am also worried about failures. I've had quite a few SD cards get corrupted in the lab and a couple fail entirely. Did your testing reveal any particular good brand? Also have you tried industrial SD cards? – fred basset May 15 '14 at 22:10
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    A handful per 200 in 3 years = typical failure rate of 2-3% within 2 or 3 years? That kind of failure rate makes me want to go double-check my backups. – Chris F Carroll Jul 19 '15 at 08:30
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    @ChrisFCarroll, I would be relaxed if it's just 2-3%. If you check out the hard drives failure rate, you will be surprised and even horrified https://www.backblaze.com/blog/hard-drive-reliability-stats-for-q2-2015 – GTodorov Apr 04 '16 at 20:42
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Don't store them below -40 °C or above +100 °C (for example, a car dashboard in some places).

You can theoretically damage them with a severe enough impact. 2000 g or more might be enough.

Don't short the pins, or use them in space. Don't use them for long term archival purposes - in 500 years several of the compounds will have started degrading and no-one will know how to read them any more.

Aside from that, I think it will take substantial physical damage to lose data in storage.

Peter Mortensen
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Colin Pickard
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    A regular SD card is not radiation hardened. The controllers are vulnerable during operation. You can transport them into space, or into a nuclear reactor core, just don't attempt to operate them there. – Colin Pickard Sep 08 '09 at 16:28
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    Suitably hardened flash memory is actually used in space all the time. – Colin Pickard Sep 08 '09 at 16:28
  • @ColinPickard, yes, it's called industrial flash storage, and it costs 10x to 1000x what normal SD cards cost. – Aaron Campbell Nov 04 '16 at 23:41