Hard drives and USB keys are different on the technology used.
Hard drives are mechanical: magnetic trays on which to move the motorized arm.
USB keys are electronic: they are mere fleas.
So who is the fastest?
In fact, everything depends on how to use them.
Hard drives have a disadvantage: It takes time to move the playhead to the right place, which takes considerable time. But once the head in the right place, the data rate is phenomenal.
The USB key, it has a very low speed compared to the hard drive. But it did not playhead to move: it therefore immediately accessible at any place of "hard".
Slow to get to the right place
Very fast transfer
Very quick to get in the right place
Reading large files will typically be faster on hard disk, because the travel time to the file is negligible compared to the time you will read the file.
Reading a large number of small files will often be faster on USB key, since much time is given to fetch the files in the right place. (On a hard disk, the 'crépitements "which means for example when the operating system starts to look or files).
It was after this observation that Microsoft has tried to set up ReadyBoost technology, but without much success.
The problem of access time of hard disk can be mitigated by regular defragmentation.
Note that the disc SSD (Solid State Disk, consisting of memory similar to the USB key), it also gets rid of problems of access time, which offers very attractive performance.
The SSD disk controllers have interlacing algorithms to the records on different chips and use a cache to reduce the problem of flow.