When copying files from my 8GB USB 2.0 flash drive with Windows 7 to a traditional hard drive, the average speed is between 25 and 30 MB/s. When doing the reverse, copying to the USB drive, the speed is 5MB/s average. I have tested this with about 4.5GB of files, a mixture of smaller and larger ones. The observations were the same on both FAT32 and exFAT file systems on the USB drive, NTFS on the internal hard disk.

I don't think I can be mistaken in saying that flash memory has a lot higher performance than a spinning hard drive in both terms of reading and writing.

For both memory types, reading should be faster than writing too.

Now I wonder, how can it be that copying files from a fast read memory to a faster write memory is actually slower than copying files from a fast read memory to a slow write memory?

I think that the files are stored in RAM before being copied over too, and there's caching as well, but I don't see how even that could tip the balance. It can only be in the advantage of writing to the USB drive, since it is "closer" to the SATA system than the USB port and it will receive data from the internal SATA HDD faster.

Perhaps my way of thinking is all wrong or it just depends on the manufacturer of the USB pen. But I am curious.

  • Some experiments you might want to try: (1) Copy from hard disk to hard disk. (2) Copy from internal hard disk to a different internal hard disk, if available. (3) Read from the hard disk, but don’t write it anywhere (e.g., by copy *.* nul). – Scott Sep 25 '12 at 23:25
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    In my experience, lower-cost USB drives have a VERY slow write speed. This is by design. Bear in mind this is not scientific. – user3463 Sep 25 '12 at 23:29

Non-rotating storage is almost always faster, both reading and writing, for random access, since the non-rotating storage has neither seek delay nor rotational latency. But when you're reading sequentially from rotating disk and writing to flash drive the rotating disk can deliver bytes at an enormous rate, while the flash drive is limited by it's relatively slow write time.

  • Chosen as best answer for pointing out the imporant difference between random and sequential. – MarioDS Sep 26 '12 at 9:40

The speedups you see in full-feature SSDs are the result of a variety of things that just aren't present in a simple USB thumb-drive.

  • Multiple memory banks, allowing parallel writes across multiple channels
  • Multiple memory chips, allowing parallel writes within banks
  • Much more sophisticated controllers

These are the things that allow flash to turn in killer write performance, and those are not present on most USB sticks. The cheaper ones don't bother to do any kind of wear-leveling, so writes will invoke the erase-write cycle far more often than a SSD.

  • Interesting answer. OF course I had a suspicion that actual SSD's were a lot more complex, but about the last thing you said, what does that mean for the lifetime of a thumb drive that is frequently used? Can you expect it to make 5 years easily? – MarioDS Sep 26 '12 at 9:37
  • @MarioDeSchaepmeester Depends on what's done with it. If it really isn't doing any wear-levelling, I'd expect the FAT blocks to fail first. If it is though, it could make it that far. – SysAdmin1138 Sep 26 '12 at 15:37

USB drives have to erase before they can write. 5MB/s is quite typical for cheap USB drives. You can find a list of drives with performance measurements on this site. 5MB/s is middle of the road.

  • I have the intenso rainbow line (8GB). Seems like I actually get slightly faster speeds than those in the table :) – MarioDS Sep 26 '12 at 9:41

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