When copying or moving files (in Windows for this example) it shows you the speed at which the transfer is currently happening. However this is very rarely a constant rate and can vary wildly from second to second. What factors are involved in affecting this speed?
In: Technology
Adding to the other answers that people have mentioned, I want to point out that the drive itself lies to the computer about where it copies.
Often what will happen is that the drive itself has a small write cache, a place where it will write files before permenantly writing them to the disk. Writing to this cache is often a lot faster than writing to disk. This saves computer resources since once that is done, the drive itself handles the rest of doing the final copy from cache to the drive, leaving the cpu to do whatever it wants. This is convenient especially because large copies are pretty rare from the computers point of view, most programs will only write small files at a time and this caching helps free resources.
If the file is too big, it will often fill up this cache, at which point the disk complains to the OS that it is copying too fast and asks it to wait while it clears the cache to disk to receive more files. This is why the first few megabytes of a file copy fast but afterwards it slows down a lot.
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