The 64G Patriot micro SD I had been using in my cell phone from mid 2014
just decided to turn itself into a read-only memory card. From what I read,
it most likely reached its maximum number of uses, as it happens at least
with some Samsung cards too. It would be to protect the card from losing
all its data, after its cells were erased "too many times" (limit number
depending on the card, and appearing to be in the order of 10-100k). And
according to Internet forums, and card reviews on Amazon, it looks like it's
getting more and more common!
A very bad point is that there were no error messages at all. I added music
files before a trip, but I had none of the new files available later so at
first I thought I didn't do it correctly (even if the transfer was fine, it
could for example have been to my card backup on an hard drive instead of
going to the actual card). Then, despite the pictures still being taken
correctly by my phone (browsing was OK, able to delete the bad ones...), I
lost all of the new ones when my phone rebooted. So they were only in a
cache memory somewhere, but nowhere on the SD card (not found by deep
recovery tools either). More fun, the older ones I deleted came back during
the same reboot...
I understand it would be bothering to have an error message at each card
access, but at least I would have known to change the card and would not
have lost 3 days of pictures! So beware...