C300mkii cfast file corruption issue – and one tedious solution

meNo shit. There I was. At the end of a long day of shooting for a commercial client with a rented C300mkii, and I get this error message: “Buffer overflow.” I tried to dismiss the error but the camera’s OS had frozen. I had to pull the battery to cycle power and restart. I hit record and same error. Additionally, another error popped up with something like “some files need to be recovered.” WTF?

My rented 256gig Lexar 3400 Cfast card was 6 minutes away from being full in slot A. I had a fresh card (same brand and capacity) in slot B, and my first thought was to switch cards. I did that, and everything was happy. I finished the shoot without further incident.

But when I inserted card A into the reader that evening, much to my surprise, my Mac froze and had to be hard-rebooted. Damn! Now I was getting nervous. This was a big project for a client who had flown to Portland from Austin, at considerable expense. I started to imagine the extremely uncomfortable conversation I was going to have to have with him.

“Um, you know that second day of shooting we did? The one where you kept telling me the shots looked so much better than our first day of shooting? Yeah, that extra day of shooting that you didn’t plan to pay for but did because I convinced you it would be worth it?”

I did some Googling, and discovered that the only Cfast cards officially supported by Canon for the C300mkii are all SanDisk cards, in capacities only up to 128GB.



I was extremely disappointed to find discover this fact, because my rental house, Lensrentals.com, had listed the Lexar drive as “works well with” the c300mkii. But who knows whether the Cfast card error thrown by the camera was related to the media anyway? Maybe it was something else. But I didn’t want to point fingers: I wanted to find a solution.

I put the bad card back into the camera, switched into media review mode, and held my breath as I tried to open the files. And guess what? I could open them! Big sigh of relief. If the data was readable on the camera, that meant the files would be recoverable. But how?

lexar3400After several attempts to do disk first aid and data recovery on the card failed with the same OS freeze as before, I got another idea. Would it be possible to copy, one at a time, the files from the Cfast card onto the camera’s SD card, which normally records proxy media? Nope. No can do. There isn’t even an option for that. But as I was exploring the C300mkii’s file options, I discovered that it IS possible to copy files using the camera menu from Cfast card A to B. I took another deep breath and gave it a try and…bingo! The file copied and the copied card was readable on my Mac.

It took me about half a day to painstakingly go through all 128 files and copy them over, but it was the best half day I’ve ever spent.

I alerted the rental house to the potential card compatibility issue, and they said they’d investigate. I still have no clear optic on what caused this data drama. But I do know a couple of things. The first one is that a problem like this has never happened to me while shooting with my Sony FS5. The second is that the next time I rent a C300mkii, you can be damn sure I’ll be sticking with the cards that Canon has officially supported.

UPDATE: After investigating, the fine folks at Lensrentals.com told me “it looks like the card was defective” and issued me a refund for the card rental.  “We haven’t had any higher issues with these cards/cameras than any other. I wouldn’t hesitate to use them again – just perhaps with a backup or two.”

32 thoughts on “C300mkii cfast file corruption issue – and one tedious solution

  1. dave yoder

    I’d like to humbly suggest the real lesson should have been to always bring enough cards to write to both cards at the same time so one will serve as a backup in case the other is corrupt. I’ve had cards die (also Lexar), unrecoverable even by the manufacturer’s crack data recovery lab. I never write to only one card unless I’ve filled them all.

    Oh, and yes Sandisk does seem to be more reliable than Lexar, in my experience and word on the street.

  2. Paul Matthijs

    I personally wouldn’t dismiss this straight away as an issue with the media. Buffer overflows sound like a software issue, so it might have been a glitch with the C300. Also, “offically supported” can be read as “we didn’t have time or budget to test more brands” or “we have a deal with this brand”. You’ll just never know.

    >I’d like to humbly suggest the real lesson should have been to always
    > bring enough cards to write to both cards at the same time

    Exactly, and/or offload sooner.

    In computer hardware circles there’s a saying: “If data doesn’t exist in three places, it doesn’t exist at all”. I know there’s almost never the time or budget to adapt workflows to that, until that one time you run into mayhem like this. No producer will ever take the risk to loose all his data if you tell them up front. If he/she does, you haven’t been clear enough 😉

    1. Dan McComb Post author

      Great points. I should probably have used a smaller card, which forces offloading more frequently. Always a good practice. Will make that three places line my new mantra.

    1. Jon Roemer

      Also, just noticed that Canon USA has added the 3500x Lexars to their list of approved media. That said – I tend to use SanDisk after having had issues with Lexars years ago.

  3. Rolf Kevin Zigler

    Rental cards are always more risky than cards you own and properly care for. Using rental cFast cards (but also just good practice):

    1) Always do a full format and refresh before an important shoot. SanDisk: http://kb.sandisk.com/app/answers/detail/a_id/17370, Lexar: http://www.lexar.com/support/downloads

    2) Never record more than 75% full. This allows better speed and better data leveling of the card.

    3) Never erase a segment during the shoot or before you offload the card. Erasing video files from the card leaves data “holes” on the card which both the FAT and leveling* have to contend with. This practice increases the chances for data corruption.

    4) As has already been discussed, use redundant storage whenever possible (write to both cards). This practice greatly improves your chances of successful storage against individual card failure.
    *(Card “leveling” is a process the card controller uses to try to write to each section of the card a similar number of times. This is necessary because each card has a limited number of read-write cycles before failure. Unfortunately, this process introduces data fragmentation as a necessary consequence.

    Full Format and Refresh tends to ameliorate some of the issue as well as correcting the fragmentation caused by people who previously deleted video segments on the card.)

    1. Dan McComb Post author

      Hi Rolf, thanks for taking the time to share your experience with cFast cards. I’ll be following these tips closely next time I shoot with that camera!

    2. Steve Oakley

      you only have to write once to a storage cell on a card for it to be validated. so you can fill it up completely, thats fine. there will be no significant difference in I/O performance after the card has been filled. in fact best things to do is to erase / over write the card before to validate all its storage since a new write requires validation dropping the I/O rate typically in 1/2 as the controller validates the info. as for fragmentation, completely irrelevant for solid state media. accessing the first or last location of storage is exactly the same.

  4. Lars Lindstrom

    Just had the exact same thing Happen to me on a commercial. 256gb lexar 3400x on a c300 mkii gave the same error and froze the os. Lexar won’t replace the card with the new and approved 3500x. Only the now worthless to me 3400x line. Good to see I’m not alone. Sandusky, here we come!

  5. xea

    two 3400x cards have failed on my shoots and even the 3500x card has failed with the “buffer overload” problem. I suspect two things here. The 3400x cards were defective. I have a feeling that it has something to do with the simultaneous recording to the SD card for proxies. When the message “maximum number of clips reached” appears for the SD card, I suspect it interferes with the Cfast 2.0 card and overloads the buffer. I noticed this happen twice already when my SD cards filled up both times with the maximum number of clips (however many they were). Obviously the cards were faulty but it might also be a firmware problem where the Canon engineers did no program for this eventuality where recording onto the SD card might interfere with the CFast 2.0 cards in the event when the SD card maxes out on the number of clips or maybe even storage space.

    1. Dan McComb Post author

      Hi Xea,
      That is a very intriguing bit of sleuthing you’ve done there. Thanks for passing that along. A workaround would be to use a large SD card so that the CF card would fill up before the SD card. Will bear this in mind next time I’m using this camera.

  6. Dan Evans


    We’ve had two Lexar 3400x CFast 2.0 cards fail on us with our C300mkii, failing to mount when put into the reader. I couldn’t repeat the error with the faulty card the first time, so I put that card at the end of the pile (i.e. last resort), and religiously checked files on every shoot. Now a different Lexar 3400x card has done the same, and the DOP failed to spot this on-set. Thankfully we’ve managed to retrieve most of the files through lots of trial and error using the Lexar recovery software.

    The advice I’ve been given is to use 3600x cards, which we’ll unfortunately have to invest in, and in the meantime shoot to dual slots at once. Quite an expense though, which is very frustrating.

  7. Todd

    I’ve had 3 sandisk card fails losing footage. Funny thing is i put the sandisks in my recovery software and the both show up as lexar. I have a feeling they are all made by the same company and “tested” by Sandisk or Lexar. Like Computer Ram is.

  8. Reuben Hernandez

    Hi Dan,

    I just recently had 2 Lexar 128 3500X cfast cards fail on me with the same buffer overflow error. The first time I was filming at 310 mbps and the second time was at 410 mbps. Not sure which cards to get now because the sandisk cards have the worst reviews on B&H. Also not sure if the issue is related to not sanitizing the cards (Lexar only makes the tool for PC and I don’t own one). Canon told me it’s the card that is failing and not the camera. My first replacement card Lexar sent seems to work fine. Have you experienced any more issues with other cards?


    1. Dan McComb Post author

      Hi Reuben,
      I don’t shoot regularly with the C300, have not experienced any further issues. I’d say stick with the cards that Canon recommends, and shoot smaller cards so that if something does to wrong, you won’t lose everything!

  9. Justin Groom

    Hi y’all,
    May 2018 update! I had this issue today, only occurred shooting 4K during a long interview session using Lexar 2500x CFast cards (no issues for past 2 years shooting shorter 4K clips). I first got the “Buffer Overload” on card A, and then switched to card B. Finished the interview with no problem and wrapped the day… only to find out later that card B wasn’t recognized my both the computer… and then the clips weren’t readable on the camera! So even though card wouldn’t mount on Mac. I opened Disk Utility, hit first aid (which gave me an error), then hit the mount button again… and it worked! I had tried Disk Utility before to no avail… but somehow hitting the first aid button (which again did nothing) and then hitting mount worked. Go figure. I still lost some data overall, but luckily also had an HD backup recording on EXT monitor. Always have a backup.

    1. Dan McComb Post author

      Thrilled to hear you found a quick solution Justin. It really does amaze me how many other people have encountered this issue. I have never once had a similar issue with a Sony camera, just sayin’.

  10. Sebastian

    This has been a very interesting post, thanks to everyone! I have recently had two lexar 256GB 3400x cfast cards fail while shooting with the C300 MrkII. Error’s in camera at the end of the card and then unable to mount and pull data. Looks like I just won’t be using the 3400’s anymore. I was curious though where in the menu for the mrkII did you find the option to transfer from card A to B? Looking all over but I don’t see it. I would love to try…thank you!

    1. Dan McComb Post author

      HI Sebastian,
      If I remember it’s a function of file saving – you tell it the destination when you copy it, and choose the second card. I don’t own a mkii, though, so I’m afraid I can open it up and be more specific than that. Rest assured there is a way to do it, though, because that’s the trick I used to save my files. Keep digging in the file menus – you’ll find it in there somewhere.

      1. Sebastian

        Found it! I was looking through the Camera menu, its under Media (which makes sense I guess!) Just select the clip and choose copy…love it! thanks for the help its working. I had the proxy files for this card so in a pinch I could use those but it feels good to be getting the 4K files!

  11. Steve Hyde

    Good discussion. A reminder that solid state media is not invincible. Best practice is to set the C300MKII to dual slot recording and offload using Shotput often. If the card clears the checksum, reformat the duplicate card and use it as a duplicate of the next. And always record proxies on the C300MKII, which is a third back up of the 24bit audio( if you record the interview in camera). I’ve been using the C300MKII a lot for the past two years with minimal problems, but I have had card errors a few times using Lexar 3500x 128. That said, I recommend the 3600x because they work in both Arri and Canon.

  12. sabah

    I´ve got this card together with my Canon C300 MK II. The card working flawlessly in the camera b ut I have problems with downloading the pictures with the Sandisk 128gb card reader to my iMac.

    I am using MacOS Sierra and it is upgraded the last time today to the very latest version…

    The computer is displaying the Sandisk card but neither PS, Bridge or LR can download the pictures on the usual way putting them in dated maps. The Bridge can not even “see” the card even if it is displayed next to the Mac hard drive…

    Is there anyone who is using similar Sandisk card with iMac and MacOS sierra can help for solve this problem

  13. Matt Crum

    Before buying a C300mk2 a couple years ago, I had experienced similar card issues with both Lexar and SanDisk cards on a couple projects using rented or client owned cameras. So when I bought mine, I decided to invest in the new Sony line of CFast 2.0 cards instead and have now shot hundreds of hours at all frame rates and data rates and have no issues at all (using a SanDisk card reader with both Mac and PC).

  14. Blaga

    Hey everyone! I have a problem and if anyone has a fix would be great! I rented a Canon C300 Mark II cinema camera and the files from one card were corrupted during transfer. I can’t get the card again or the camera and I need to find a data recovery file that would fix the MXF files from my drive. Do you know which software could help? Thanks a million!


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