I have a 1XD and mostly shoot weddings. During a wedding I had a Lexar Professional 128GB 800x card and a SanDisk 64GB Extreme Pro 90mbs. I was shooting RAW to the Lexar and JPEG to the SanDisk. About 2000 pictures into the day I went to review a series of images I had just taken and could not because the camera was busy writing to the cards. I do not spray and pray so I was confused as to what was happening. The buffer never did clear and I took the risk of trying to shut the camera off. Next a spinning graphic along with the message that the camera was still trying to write 14 files to the cards came up on the LCD screen. I let it be for about 5 minutes and then felt that I had to move on with the day and wasn't really willing to stop using my 1DX. I took the battery out and put it back in and was able to review the images and sure enough I had lost the 14 pictures it was trying to write. It wasn't a huge deal because I had already retaken them with my 5D3. I was able to take pictures again but I noticed that the red buffer light stayed on continuously. I found out that the 64GB card was causing the issue so I swapped it out and continued without any issues. My question is that do you thing the issues was with the camera or card(s). I feel like I shouldn't have these issues with expensive cameras and nicer, newer cards. However I know that crap happens.