I was using my one year old eyefi card in the M for starters. I rarely transfered off the card when I used it on the 5D, it was only for wireless transfers. I've had the card error out the 5D at times, but not fail at transferring data.
This pertains to a movie file shot on the M. WHen I view the movie on the M on the same card, it plays throughout. When I try to transfer the data to my computer via wire, it fails, with the mac os saying that it can;t complete the transfer because the data is unreadable. this happens at 1min 37 seconds into a 7 minute program.
Next I'm going to try transfering wired fromt he camera itself since it can play the whole clip through.
Again, the movie plays fine on the M, but fails at copying wired to the computer.
Prior to this i had the card unmount repeatedly from a $9 reader that I got with the camera via B&H. the reader had many good reviews on B&H:http://www.bhphotovideo.com/c/product/502433-REG/GGI_SDHC_Secure_Digital_SD_SDHC.html
This has only happened with EOS M files but I've only had the camera for a couple of days and the card works fine in reading and recording files onto the camera natively. When I used it on the 5D, i only used it to wirelessly transfer and never reallyused it on a reader.
The card was low level formated on the M prior to use. It wasn't near full at all.
It has the latest firmware from Eyefi.
Thanks for any feedback. I'll keep the gang posted.