Recently, following exposure of fake SanDisk cards, most of the byers became cautious about this. But some fakesellers didn't gave up - even today ebay filled up with such cards.
They just learned from mistakes - they sell 4Gb cards carefully disguised as 8Gb, as well as 2Gb cards disguised as 4Gb ones.

This is how it can be possible:
SD card consist of two types of micro chips: one - controller, another - flash memory.
Controller chip used to couple sofisticated serial SD interface of card reader, HPC or mobile with all it's error checking, speed settings, etc. to parallel and relatively simple interface of flash memory. Most types of controller chips can be used with any size' memory chips, but controller itself should be correctly programmed by card's manufacturer to access it's attached flash memory. And this programming includes size of flash memory chip and how to handle it!

Shortly, what all this about: Some greedy fakesellers got reprogramming equipment and software in their hands and started internally re-marking (yes, they can be formatted and writen up to their "full" capacity because controller always says "OK" then you're writing something in up to it's declared size, but nothing actually got written above it's flash chip's actual capacity) some cards as an at least twice larger ones.

How to check your card: write to it as many files as possible (up to its maximum size or at least more than half, the more is better), I suggest using large video files, REMEMBER which files were written last, SAFELY eject the card, then re-insert it and try to play or othervise open your last-recorded files. If they're failed to open or contained only zeroes, you have a fake card, sorry. Now I'm searching for software to re-format such cards into it's correct size, but no success.


Surely, these sellers should refund at least 2/3 of money or be reported to the anti-fraud service or at least be left with negative feedback with description of their crimes.
And at  last, foto of one such card (unfortunately I got two) - it has a 4Gb flash chip inside, do google on it.

Best regards.

As an update, I'll post a message from someone who wishes to share his/her bad expirience with ANOTHER type of fake cards with the same exterior, invented by our "friends" from southeastern corner or Eurasia. Here's the mesage:

I read your guide about fade SD card. I received a fake 8GB SDHC memory card from ebay last week. It is faked in a more advanced way. After I wrote more than 4GB in total, the new file would steal space from and hence corrupted the existing files. Stolen space returned 0xFF on read. You may find the file just written OK (unless itself is greater than 4GB in size) but the rest may have been corrupted. We would need to verify all the files after going above 4GB to be sure.

This fake is more dangerous as it may corrupt our archive without notice.

I am sure that the only purpose of such a design is to trick buyers. Nobody would find it useful in any ways.

You may want to update your guide. The fake looks exactly like your picture externally.

When I complained to the seller, there were no replies. Then I raised the case to paypal, I got a full refund from the seller without saying a word.

However the bad feedback I left on the seller somehow also disappeared."

So, starting this moment, SIMPLIEST ONE AND ONLY ONE WAY to found that is your card good or not, is to fill as full as possible your card with data, again I'd prefer large video and audio files, then safely eject card, then re-insert it again and try to play ALL written files, starting from first written one, of course checking the contents. If your card is a  "new-way" faked, you'll get error or wrong content right from beginnig of the media file.

1. Fill full, remember first and last written.
2. Safe eject than insert.
3. Check first written, than check last written, than all other files, both for content and playback errors.

Please never say fakeselellers about this guide, as they can invent something new which can not be detected easily.

Advanced users can use "geeky" way of  "fc.exe" dos prompt command. This one cannot be tricked at all.