This has happened to me a couple of times with off-brand cards, here are my experiences:
32GB Wintec Filemate Professional Class 10 SDHC, K-x
A) The card was used as-is out of the box, no formatting applied
B) Gradually filled up the card to more than 8GB
C) Turned on the camera, took some photos in burst mode until the camera stopped to process the photos, turned off the camera in hurry, then turned it on again: photos were gone
Solution: copied all the photos off the card onto my computer hard drive, then put the card back into the camera, formatted the card in-camera, worked fine
32GB PNY Professional Class 10 SDHC, K-5
A) Before using the card, formatted in-camera
B) Gradually filled up the card to more than 8GB
C) Turned on the camera, took some photos in burst mode, waited until the photos were saved and the camera was ready, turned off the camera, turned it on again: photos were gone
Solution: took the card out, blew some canned air (gently, from a distance) into the card slot, blew canned air over the card itself on the metal contacts, put the card back in, worked fine
Not sure what causes the problem, but it seems that in general, off-brand cards larger than 8GB are finicky to deal with. Before this happened I didn't realize that cards could be formatted in the camera (apparently this is better) or while hooked up to a card reader on your computer (apparently this is not as optimal). I have 12 different SDHC cards, and I have never had a problem with the Panasonic or ProSpec (Calumet's brand) cards. Also have never had a problem with any cards that are 8GB or less in capacity, off-brand or no. Finally, when this first happened I was in a hurry, I am not sure if I was being careful about waiting until all the photos taken in burst mode had safely been stored before turning off the camera.
Based on experience I would say that if you buy a cheap high-capacity, high-speed card, make sure to format it in the camera first, then test it by copying out 8+ GB of photos from your computer, put it in the camera, and see how it does. Test the card by taking photos in burst mode and see how the card behaves when it is at near capacity. And make sure to blow the dust out of the card slot.
Now whenever I copy photos off a card I always format it in-camera before filling the card up again. Have not had any problems since, have filled up the 32GB cards more than once. I also keep my camera firmware as up-to-date as possible, as I believe one or more updates have addressed some related card capacity issues. (Though not this issue in particular, as far as I know.)
One thing I did not test (but I have a suspicion happened): whether the photos that I lost were actually saved on the card. The recovery software I use (Recuva) is handy for things like getting back pictures accidentally deleted. It would be interesting to scan the card with Recuva or other recovery software and see if the pictures were saved, but something went wacky with having them show up as saved. I am willing to bet that the pictures are in fact saved on the card when this happens. Though of course, if you take more pictures and they overwrite, or if you format the card, that data is gone.
In general, it would be nice to always buy Panasonic cards (my favorite brand) but a few times the off-brands were so cheap it was hard to pass up! Illogical I know, but there it is. If buying a card on vacation or for a photo shoot or in some other situation where playing around and testing it first was not an option, I would probably avoid the off-brands no matter how cheap.
