The above might seem far fetched, but amazingly the same con trick is taking place with SD memory cards being sold by vendors on Amazon and eBay. A memory card is described as having, say, 128GB capacity, but when you start using it, it fills up after maybe just 8GB of data or files start vanishing. It's been going on for years, has been highlighting by thousands of people and on social media, yet still the cards are permitted to be sold on well known websites. So how can you protect yourself? What are the signs to look for?
Firstly, remember the old maxim of 'if something looks too good to be true, it probably is.' Price is the first giveaway - these products will be a lot cheaper than the real thing. For instance, at the time of writing, brand name 128GB cards retail for around £25 upwards in the UK, so if anything is significantly cheaper than this (say less than half-price) you should immediately be suspicious.
Next, look at the description of the item. A number of standard phrases are used by the vendors, in the belief that these will absolve them from any accusation of fraud or misrepresentation. The most common one is the claim that the card has been 'upgraded' and therefore "speed and capacity may vary with each card". No. You cannot 'upgrade' memory cards, which are manufactured in specific sizes, any more than you can 'upgrade' a 330ml can of cola to hold 2 litres of cola. However, speed certainly will vary: ranging from slow to very slow. Generally speaking, the brand names (SanDisk, PNY, Toshiba, Samsung etc) get the faster chips and the slower and reject ones end up in the obscure, unbranded and generic cards. In some cases, the vendor may even say something like "they may experience read/write errors once max capacity reached" - interesting use of the word 'may'! Other things to look out for are: a photograph of a memory card and/or packaging without any recognisable branding information; use of the term 'generic'; a warning that it may not work with all devices; the phrase 'straight from the factory'.
The reality is that these are invariably slow, low capacity cards that have been hacked to appear to be bigger than they are. This is done by using a non-standard formatting utility, which in essence lies about the formatted capacity of the card. If the first thing you do with one of these cards is format it in Windows or with the Mac Disk Utility, it will revert to its real size. At which point you have a slow card of dubious reliability.
Why the big name websites continue to allow people to sell these cards through them is an interesting question...