Strange performance differences in read/write from/to USB flash drive

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

My answer is that the chosen answer is incorrect. I am having same issue except in this case I used single 800 MB file. Reading from USB HDD was 80 MB/s, then I overwrite(replace in windows terminology) the file with the same data and this speed was 15 MB/s. The explanation provided in the chosen answer does not explain this difference. Another anomalous thing is I had previously written to the HDD approx. 80 MB/s as well so it had worked at full speed before. If there is doubt about contiguousness, sysinternals provides a tool contig to deframent the single file. However since the read speed was 80 MB/s that already proves the area of disk the file was at was not fragmented, otherwise both read and overwrite speeds would have reflected this.

Answer 5

I have been testing that too. I´m copying SDD to USB, HDD to USB, USB to USB and it is slow. Using linux pv < /dev/sdb > /dev/sdd 81.1GiB 4:59:40 [11.5MiB/s] [======> ] 69% ETA 2:13:20. But copying an USB to a hard disk using DD linux 117GiB 1:28:51 [23.6MiB/s] is faster, it took me about an hour and a half. A little more than twice as fast than copying to another USB.

I also found that depends also on the bus 2.0 or 3.0 used and then brand of the USB. Cheap USB on 2.0 ports will go down to 5MiB/s, more expensive USB brand names, can go up to 10 to 12MiB/s copying USB to USB.

Conclusion: Depends on several factors, the PC Bus, the version of port, the brand of USBs and combination of all. Can get worse if you copy the disk image in to a HD folder and then try to copy to a USB. I think is due to de internal bus transfer. USB to USB was faster on my computer. That's my report. The best way is to got to different PCs and test, and replicate on the one that runs faster.

Yes, it was faster from SSD to USB, its faster to copy from USB to HDD or SSD. Really slow the other way aroud. USB writting speed is slow.

FYI: Im replicating over 100 USBs sticks for event, so I was able to replicate exact tests. AH, forgot to mention, also used WIN10 with RUFUS it only wrote up to 5.5MiB/s so there was no improvement.