You need to show us the command exactly how you wrote it, not a copy/paste from some random website, that command is for Ubuntu, not Debian , yes I see you say "adapted", but that could be anything,...are you sure you didn't down load a Ubuntu image, ?
I was just reading earlier today about some others with a similar problem, apparently there are some corrupted images floating around, but since it was for Ubuntu, I did not pay any attention, and did not save the links.
Also,
I do not know whether this "flushes the copy buffers".
"this" ?, it all depends on how you actually worded/wrote the command, and we do not know that either.
You really should read the 'dd' manual, before using the command you could very easily trash you HD just copy/pasting from some random website,
Using dd, on the other hand, can make perfect byte-for-byte images of, well, just about anything digital. But before you start flinging partitions from one end of the earth to the other, I should mention that there's some truth to that old Unix admin joke: "dd stands for disk destroyer." If you type even one wrong character in a dd command, you can instantly and permanently wipe out an entire drive of valuable data. And yes, spelling counts.
"instantly" is a bit of a exaggeration, the 'dd' command is slow, again depending on how it was written, but it can make it almost impossible to access any data, in just a few seconds, or less.
Check and make sure you really downloaded the Debian 64 bit ISO, as well, maybe you got a 32 bit ISO.
I think I would just wipe the usb stick, and try again, if I was not sure about the image, I would download another, and be more careful about which image I selected.The netinstall
images are small enough to be reasonable, and don't take long to download at all.
I have never used a 'dd' command that was even close to what you show. You might try using the example in the manual , it is easier to adapt and works well.