hi there
I have been using the 5D for a week now, and bought a Transcend 16GB 133x which I though would be fast enough for videos.

now, every now and then the screen showed the "buffer", which then disappears quickly.
but increasingly, as I get to fill the card (I have maybe 4GB left), the buffer fills up and the video stops completely, as to indicate the card is too slow and it's "chocking".
it happens at least once every three times.

that's what I fail to understand - if the card IS too slow why doesn't it happen all the time?
what cards are secure enough not to have this issue?

I have the same card and have never had any problem.

that's precisely my point.
could it be my card is defective? is there a way to check this?

I've had the same issue with a Kingston 133x. I don't think your card is defective, it's just slow for video I believe.

It all depends on the scene; the more complex it is, with highlights or a lot of details in shadows as you film, then the 16GB 133X cards will sometimes cause the 5D to 'buffer' as it stutters to keep up. I have numerous Transcend 16GB and Kingston 16GB cards and they ALL have this same problem.

However, my Transcend 32GB 400X UDMA cards never cause the 5D to 'buffer' and always continue to run smoothly on any scene that I film, even if I'm working it hard with combined stills & video. That is why I much prefer to use the 32GB 400X cards for prime shoots and keep the 16GB 133X cards for back-up.

I use these cards with a Nanoflash and can record 100mbps footage no problem. I do not think the card is too slow for the ~40mbps Canon codec. On the reverse side, I have recorded video on the 5D with my old 2GB still use CF cards and it recorded fine.

bought a Transcend 16GB 133x which I though would be fast enough for videos.

now, every now and then the screen showed the "buffer", which then disappears quickly.
but increasingly, as I get to fill the card (I have maybe 4GB left), the buffer fills up and the video stops completely, as to indicate the card is too slow and it's "chocking".
it happens at least once every three times.

That is a definite symptom of a card that cannot keep up with the camera.


that's what I fail to understand - if the card IS too slow why doesn't it happen all the time?
what cards are secure enough not to have this issue?

The card is "marginal", many use Transcend 133x cards and feel they work fine, but I've seen references to 133x being "marginal" for both the 5D MkII and the 7D, and I've seen Transcend mentioned in some of those reports. When the 7D first hit the scene I saw several recommendations that even in SanDisk 30MB/s might not be quite fast enough although some were posting those worked OK for them.

I ordered a couple of 8GB SanDisk 45MB/s CF cards for my 7D and when those were discontinued got a couple of SanDisk 60MB/s cards; the SanDisk brand seems to exhibit far fewer problems than all other brands except maybe Hoodman RAW (never saw a single complaint on that brand).

Transcend enjoys a better satisfaction level than the other "bargain" brands, but there are still just enough problems reported to show they don't have the QC SanDisk does.

I've followed this issue since the T1i came out.