They're likely a cheap SD card inside the case with firmware that pretends to be 10TB. When you use them, they appear the correct size, but once you go over the size of the actual SD card (e.g. 256GB) you overwrite existing files and corrupt them.
You can tell that they're fake as the price is unrealistic.
I just searched for the prices of 256GB SD cards and they're about 20$. At this point I guess they put 128 or even 64 gb SD cards, otherwise it makes no sense...
Yeah, they go for the cheap ones so likely not 256GB. The only advantage of the bigger sizes is that it takes longer for the user to corrupt their files which is probably a disadvantage for those users.
Another way to determine if you've got a fake drive is that the I/O speeds will be much slower than a usual SSD.
You can tell that they're fake as the price is unrealistic.