Virtually all of those cheap Sony memory sticks (normal and duo pro) being sold on Ebay and Amazon Marketplace are counterfeits. As the sellers of these items are trying to out-do each other on price, the prices for these memory sticks has gotten ridiculously low. 1GB = £30, for example. A real 1GB memory stick costs more than that from the factory! Although a counterfeit labelled as "1GB" may not actually contain 1GB of free space... I'll write about that in a minute...
The market is flooded with counterfeits bearing the Sony logo, imported from Taiwan and Malaysia, which are very hard to spot. They even come with gold holograms, warranty cards, etc. Previously, the counterfeits weren't sold with holograms, but now the manufacturers have cottoned on. When a trained Sony technician has difficulties in spotting the difference, the average customer stands no chance!
At first sight, a 1GB memory stick for £30 or less may seem like a good deal. It's not, because what you'll receive isn't actually a device with 1GB of genuine storage space...
The counterfeit packaging says "1GB", and when you put the memory stick into your PSP (or mobile phone, camera, etc), the device will also show 1GB of space. The trick here is very simple. What the counterfeit memory sticks actually contain is a smaller generic chip (lets say 256MB or less) which has had a higher number (512MB, 1GB, etc) hardcoded into its circuitry. As you can imagine, it's not very difficult to do these days.
Counterfeit memory sticks will store smaller files without any problems. However, if you're wanting to store something such as a large movie file (600MB+), that is when you'll start to run into file corruption problems (because that much space doesn't REALLY exist on a counterfeit memory stick).
I've included some links below which give more details & explain how to spot counterfeit Sony memory sticks (photographs included).
The safest thing to do would be to buy directly from Amazon, Play.com, Dixons, Scan, Comet, etc. You'd be paying more than you would be with a private seller, but at least you'd be getting a real memory stick with a real amount of storage space on it!
Oh, and don't be fooled by those Amazon Marketplace sellers with fancy logos for their usernames. It means nothing! They're no better than the sellers with the text usernames. I bought a Sony duo pro memory stick from a (fancy logo) seller on Amazon just before Christmas. I thought the price was reasonable, not too low... and the merchandise still turned out to be counterfeit! I guess the only thing to do is to leave negative feedback (for Ebay sellers) or '1 out of 5' feedback (for Amazon sellers) with a comment like 'sold me a counterfeit sony memory stick - reported to Sony UK and Amazon' in all capital letters. And whatever you do, don't be coaxed into removing the feedback!!
Here are those links:
http://www.trademe.co.nz/Electronic...on-47863575.htm (has photos)
http://www.forum.renoraine.com.my/viewtopic.php?t=43 (has photos)
http://www.mxphone.com/show.php?no=913 (has photos)
I hope this helps someone.