OK, thanks for all the suggestions. While computer reformat might just make it work, that won't help me in the field. I will call B&H tomorrow and see what they suggest. I'm inclined to believe there's something fishy with the card.
If you are worried in the field ... you have 2 options.
1. buy more cards (which is what I've done)
2. erase all photos on the card, instead of formatting in camera
BTW, my recommendation ... if you don't have to, don't format the card while doing a shoot
Strange thread. I've used Extreme III cards, 2, 4, and 8GB in my 40D, and the Tammy 28-75 - never a problem, and also use the same with my 10D, 50D, and 5D. Not much help, I know, but just another data point.
Where did *I* say it was related to the lens?
None of the other replies mention a lens and I was using a Canon 35 f2
You cited two threads from other forums, and stated your conclusion that "it wasn't the camera" implying you felt so because of what was in those threads. The threads you cited stated that only certain lenses caused the problem.
I suspect it is the camera and would arrange a Canon fix. Send cards and maybe lens also. I have used the gambit of different size cards in my 20D and 40D with no problems. I also use Tokina lenses sometimes and cannot see how it could possibly be the Tammy lens. If you do not have a battery issue, showing empty when it should say full, then I suspect camera body only.
I have tested SanDisk Extreme III 4GB (30 MB/s) in my 40D (1.08) and I could not recreate the problem.
Apologies for the double post, but just in case you didn't read the other 40D thread:
Could you please try taking the lens off, the card out and most importantly the main battery out for a few seconds (not the backup one) with the camera switched off, then put them all back in and switch on. Before you even get to thte sensor clean you should get !cannot create folder! on the rear LCD and the viewfinder says FULL, even though it isn't.
It certainly seems from my tests that this only happens with an Extreme III 4GB with 30GB written on it (hence my feeling that it's not the camera)
David not sure if the 40D is at fault here, but I find the 40D is an odd camera with some strange software issues in certain units
For instance my 40D (like many others, but not all) will not last more than about 30-40 shots without the battery being reported as "empty" (requiring the camera to be switched off an on again). Sometimes a fresh battery is immediately reported as "empty" as soon as it is put in.
Also I found even on Continuous mode, when the filename counter clicks over 9999, it will continue into a new folder (e.g. 101_) from 0001 again like most cameras, BUT then when a different CF card is inserted, it goes back to folder 100_ and starts naming the files from some random number like 9500 or something. Very odd.
I wonder if certain batches have an inferior connection somewhere which explains these strange anomalies and incompatibilities.
I have been very pleased with my 40D and did not have (touch wood) a single issue with my camera. I realized that I am becoming paranoid about buying new equipment and do not really want anything "new and exciting" anymore. I must be getting old.