My windows 8 slows down while copying data even when it is plugged to a USB 3.0 port. As soon as the copying starts, it gives a speed of 9mbps and sometimes goes up to 99mbps. But then suddenly the speed decreases directly to 0mbps. Why?
-open device manager
-scroll down to universal Bus controller
-Uninstall Generic USB Hub (your mousse and keyboard will be deactivated, so schedule a reboot)
check on your BIOS if there is BIOS optimized settings and load them
perhaps your antimalware is doing a simultaneous scan of the external device
This is likely due to the quality of the USB controller on the destination device. In particular, USB flash drives are of highly varying quality. A USB drive is a complete computer in itself that manages the communication over USB and the actual reading/writing of data to the flash media (true for magnetic media too). The speed variations usually seen are where the computer on the USB device (aka the controller) buffers incoming data in its own RAM while it writes data to the flash media much more slowly. This is where the speed ratings matter when you buy the drive. Another speed factor is how full the destination device is as the controller attempts to "wear level" the use of the flash. Flash media has a limited number of write cycles before wearing out so the controller attempts to even out the number of writes to each part of the media by moving more static data to different parts of the drive. Thus an individual write might turn into a read+write+write which is a lot slower than a single write. If the controller is doing a lot of wear leveling then the incoming buffer fills up and won't empty for a long time yielding the 0MBps you see. High quality controllers can read and write in parallel if the locations don't overlap while low quality controllers have to wait for all writes to finish before they can read any data. So a low quality controller can block for a long time if it has queued a lot of writes then has to wait for them all to finish to read in the wear leveling step. I've seen cheap devices block for a minute or more when this happens.