I was slightly skeptical at first. I only purchased the SanDisk 400 GB Extreme card after seeing the positive reviews here, along with knowing that Amazon would also be fulfilling the order.
However, I still wanted to be sure that the SD card was legitimate before using it to film 4K H.265 footage.
I tested the SD card using H2testw (v1.4). I used the SD card slot of a Surface Book 2 with a BaseQi SD card adapter. The test results were inline with real world expectations (the computer was also being used for other projects whilst the test was in progress).
I also compared the SanDisk 400 GB Extreme to a SanDisk 64 GB Extreme and SanDisk 128 GB Extreme Pro I had purchased from a reputable local retailer. The fonts and logo all appear to match perfectly and the physical layout of the pins and memory chip match exactly, and the text and serial number layout on the back also match.
The warranty details listed inside the packaging were also for my region and contained correct contact information.
It's fair to say that these cards are genuine and you should be confident in your purchase.
Below are the H2testw results: Warning: Only 375558 of 375560 MByte tested. Test finished without errors. You can now delete the test files *.h2w or verify them again. Writing speed: 78.7 MByte/s Reading speed: 84.7 MByte/s H2testw v1.4