USB 2.0 mass storage bulkonly protocol maxes out around 40MB/s due to protocol overhead and hardware limitations, and in practice it's closer to ~35MB/s.

https://superuser.com/questions/317217/whats-the-maximum-spe...

...which still gives ~2h as the amount of time taken to transfer 256GB. My suspicion for the slowdown (to around 8MB/s?) is the flash controller is doing read retries and applying ECC to compensate for retention failures.