Four observations:
1. The fsck.exfat command worked. So obviously the mkfs command and kernel aren't
seeing the wrong state of the card's physical lock switch. It sounds like a weird bug
that it then can't be mounted. Maybe try an explicit mount -t without -o rw. I'd
think if it's a bug it'd have been found about an hour later though, pretty
significant bug if it's a bug.
2.
Aug 25 20:24:27 localhost.localdomain kernel: mmc0: new ultra high speed SDR50 SDHC card
at address 0001
SDHC cards are supposed to be formatted FAT32. And SDXC is supposed to be formatted exFAT.
That's per the SD Card specs anyway, which is how any camera supporting these types of
cards should work.
3.
Last, and actually most important, practically obsoleting the above two observations: this
card is being used in a camera? The camera must be used to format it. That's what the
camera manufacturer and the SD card manufacturer will tell you. The interoperability is
*so bad* with SD cards, that they will both tell you that corruption and even camera
malfunction is expected if you don't format the card in-camera.
Further, it's well understood by pros and the amateur (the classic connotation of the
word implies someone more serious about their interest than even a pro) that the camera
image delete option shouldn't be used. Take your shots, fill up the card or get to a
good break point to swap cards. When sucking the images off the card with a computer, suck
all of them off, back them up, then go through and throw away the junk photos you
don't want. Reformat the card in the camera.
4.
Best quote about SD Cards ever: "You aren't storing your data, you're storing
a probabilistic approximation of your data."
https://www.youtube.com/watch?v=CPEzLNh5YIo
Quote is somewhere around 3m30s; including the point that flash memory is unreliable, and
lots of other cool observations. I don't know why these things are called secure
digital (SD) after watching this.
Chris Murphy