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I have always believed in keeping magnets and data storage devices far away from each other. Friends say I'm excessively cautious about it, but I seem to have a lot less data loss than they do!

As cell phones have gained popularity, so have cell phone cases with magnetic clasps. Often these clasps are positioned near the center of the phone, in close proximity to where the microSD card is located in the phone.

A friend has had two genuine Sandisk microSD cards (purchased at large brick and mortar retail stores) fail in their phone. They keep their phone in one of those cases with a magnetic clasp.

Is it likely that the magnet is causing the loss of data, or are such magnets much too weak to result in affecting the data on a microSD card?

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These magnets are orders of magnitude too weak to damage or even influence a card. You need several Volt to change the state of a flash cell, and one of these magnets won't have the power to do that.

The easiest way to "prove" this is to look at the internal storage of the devices, which is usually using flash technology as well. The internal storage is not damaged by these magnets, otherwise the phone would stop working. SD cards are not susceptible to magnetic fields any more than other electronics, so a magnet strong enough to affect an SD card would affect other electronics in the phone as well.

A very likely reason for failure of flash storage is having bought a counterfeit flash card, which is designed to break as soon as a certain amount of storage is written to it, because these are very cheap to manufacture. On sites like Ebay and Amazon there are huge amounts of these cards on offer - the largest microSD card by SanDisk is currently 200GB, yet the aforementioned sites will happily sell even 512 GB Sandisk cards that don't exist. There are also other causes, such as an erroneous controller for the card in the phone.

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  • $\begingroup$ Thanks! I edited the original question to indicate that the cards were purchased at major brick and mortar retailers. There are quite a few stories about counterfeit cards sold online! $\endgroup$ – RockPaperLizard Sep 9 '15 at 8:02
  • $\begingroup$ @RockPaperLizard Thanks for that. I remember there was a fix at some time to the raspberry Pi which was supposed to reduce the SD card failure rate, but I can't find it due to internet noise. The point is that the device can do many things to increase the SD card failure rate. $\endgroup$ – Peter Sep 9 '15 at 8:07
  • $\begingroup$ @Peter: Flash memory can't be written to too often. Changing the same file/flash cell again and again will wear it out. SSDs and today's better SDs distribute write operations over the entire device to circumvent this. Cheap SDs don't. And this is not possible if the drive is almost full. Using SDs as hard drive e.g. for a Pi means lots of write operations. One can reduce the write operations by tweaks, but also a card with much free space and of good quality helps. $\endgroup$ – sweber Sep 9 '15 at 8:25

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