The sudden unmounting and/or sd card not being recognized by the S4 and other devices as mentioned is a known issue that is usually SD card related. Sometimes it's just a matter of reformatting your card to Fat32 if your ROM cannot read exFAT cards.
But if that's not your issue or you already reformatted your card to the appropriate format, the SD card has failed and warranty/replacement is suggested.
Can happen to SanDisk cards in addition to other manufacturers.
You may hear this issue being associated to SanDisk cards if only because they are probably the most counterfeited card out in the market. But the issue can occur to non counterfeit cards also.
To reduce your chances of having theses issues happen, always get your cards from reputable sources, in blister original packaging, etc. And if the price looks too good to be true, it most likely is. Even if you take the above precautions, you can still have issues.
Always back up your data for that 'Just in case moment' in case stuff happens.
Purchasing a card whose Manufacturer and/or Seller has an excellent return/exchange policy is a plus.
Having a spare SD card available can help you troubleshoot what your issue may be regarding a defective card, formatting issue, device issue, etc.
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