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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?

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There is contact wear concerns also...superuser.com/questions/405942/… –  Moab Mar 28 '12 at 22:24
    
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. –  user167860 Oct 26 '12 at 9:33

6 Answers 6

up vote 57 down vote accepted

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.

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+1 for lava.....I always wondered whether my pictures would be safe in the volcano out back –  Babu Aug 3 '09 at 1:04
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If I only had a nickel for every time I lost an SD card to lava flow... –  Kevin L. Aug 3 '09 at 3:11
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One of my SD cards has been through 3 cycles, wash cycles that is, in the washing machine and works fine...and cleaner too may I add. They are really tough. ;-) –  GeneQ Aug 3 '09 at 3:19
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does not work in lava? i want a refund! –  icelava Aug 3 '09 at 11:55
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Babu's future amazon review - "one star - I took this SD card into the volcano to take pictures with and when I came out the SD card no longer worked! JUNK DO NOT BUY!" –  Nathan Adams Sep 15 '12 at 16:39

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.

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What's the problem with using SD cards in space? Not that I plan on trying anytime soon, just curious... –  Yuval Sep 2 '09 at 19:35
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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 8 '09 at 16:28
    
Suitably hardened flash memory is actually used in space all the time. –  Colin Pickard Sep 8 '09 at 16:28

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.

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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.

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nice writeup. Thanks for sharing your knowledge. Can you add a paragraph on how we can 'refresh FLASH memory data periodically'? Thanks. –  shellter May 4 '12 at 4:00
    
This is interesting. I have not heard of this. I would also like to hear more about this recharging procedure. Or is there anyone else that can back up this statement? –  zingle-dingle Jun 21 at 4:23

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.

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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 at 22:10

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.

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