I've just bought a SanDisk Ultra USB 3.0 64GB flash drive. The spec says that the drive's max read speed is 80Mb/s.
I only have a USB 2.0 port, and I've made these tests:
- Writing a 4GB file on the (freshly formatted) flash drive had initial speed of 20Mb/s, then after a few seconds dropped to about 7Mb/s.
- Writing the same file to a WD Passport USB HD connected to the same port had constant speed of 20Mb/s.
Should I suspect that the flash drive is malfunctioned or there might be a legitimate reason for this?
Best Answer
You cannot really draw a conclusion about write speed from read speed. Usually those tend to differ about some magnitudes and vary a lot between different devices.
The fact that you observe a drop of speed after a short time when writing to your USB key is most presumably due to your operating system writing buffered: For performance reasons your OS does not write directly to the USB key but to a so called buffer cache, assuming that it isn't really necessary to write the files immediately. In most cases, it would be perfectly fine to cache writes and do the real writing in the background later (that's perfectly true for hard disks, but not necessarily for USB keys). The drop is caused by bulk-writing bigger data and your OS decides that it shouldn't write to the buffer cache further.
This also is one cause that you need to explicitly eject removable devices before removing them physically.