Effects of magnets on memory cards 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?
 A: 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.
