Recently had the same "Card cannot be accessed. Reinsert/change the card or format card with camera" message on my 650D (with Meike battery grip) twice.
Both times the camera was switched on and several photos had been taken already.
The SD card was a SanDisk 64 GB Extreme (90 MB/s) class 3.
On site there was nothing I could do. So I removed and locked the card, and replaced it with a 32 GB card without ML.
First time I could recover all photos with Recuva from Pirisoft (https://www.piriform.com/recuva). This is freeware.
After this I formatted the SD card with SD Card Formatter (https://www.sdcard.org/downloads/formatter_4/index.html). This little freeware program will restore the hidden partition on the SD card also in case it's lost.
Than I added ML to the SD card again.
Two weeks later it happened again. Same SD card.
This time I wasn't able to recover my photos, but a quick format first did the trick.
After this I could recover all photos with Recuva again.
Since the second 'crash' I haven't use ML anymore and use the 32 GB card only.
I'm still not sure what caused the error, but both times the card was in the camera, camera switched on and in use.
Both times the camera was switched on and several photos had been taken already.
The SD card was a SanDisk 64 GB Extreme (90 MB/s) class 3.
On site there was nothing I could do. So I removed and locked the card, and replaced it with a 32 GB card without ML.
First time I could recover all photos with Recuva from Pirisoft (https://www.piriform.com/recuva). This is freeware.
After this I formatted the SD card with SD Card Formatter (https://www.sdcard.org/downloads/formatter_4/index.html). This little freeware program will restore the hidden partition on the SD card also in case it's lost.
Than I added ML to the SD card again.
Two weeks later it happened again. Same SD card.
This time I wasn't able to recover my photos, but a quick format first did the trick.
After this I could recover all photos with Recuva again.
Since the second 'crash' I haven't use ML anymore and use the 32 GB card only.
I'm still not sure what caused the error, but both times the card was in the camera, camera switched on and in use.