A. USB “keychain” flash drives are slower because the flash drive has much poorer writing performance than an external hard disk drive. The real world speed of writing depends both on the performance of the interface (USB in this case) and the write performance of the storage medium.
Attached by USB 2.0, an external hard drive can reach about a 30–40 MBps writing rate, limited by USB 2 transmission. On USB 3.0 (USB 3.1 Gen 1) and USB 3.1 Gen 2, hard drives will hit the maximum writing rate of the spinning hard drive, which is about 100 – 200 MBps depending on the drive.
Consumer USB sticks (on USB 3) can do 10 – 45 MBps sustained write speed. That’s down to the low cost NAND Flash chips, the rudimentary controller, and the inefficiency of writing NAND flash memory in general. Flash memory cannot be simply written byte by byte, it has to be written in relatively large blocks. If there is any data on the drive already within a block, then the whole block has to be copied to temporary storage, then erased, then merged with the new data, then written back to the block of flash cells. There are some workarounds to shuffle pages within blocks, but in USB flash sticks the controller is not sophisticated. All this means is that writing to flash takes much longer than it should https://en.wikipedia.org/wiki/Flash_memory#Block_erasure
Single large files will write much faster than many small files. A freshly formatted stick will write faster than one that is badly fragmented with existing data. USB flash drive – Wikipedia
There are faster, more expensive sticks out there, that can do up to 100 MBps or more peak rates on USB 3 User benchmarks, but watch out for the random small file performance. See 639 USB Flash Drives Compared
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