I was just copying a 4GB file to an 8GB USB stick formated as NTFS.
For the first second of the write, the data was being copied at over 100mb/s, which surprised me, but then it gradually slowed down to under 10 mb/s, which is the normal speed.
I think I've witnessed this fast beginning and gradual drop on hard drives as well, what is it? Is there a way to keep it up at 100mb/s? :-P
Nope. It starts out fast initially because Windows caches disk I/O in RAM. What you're seeing with the fast transfer rate is the data being read quickly from the hard drive and into memory. When RAM fills up, the transfer speed becomes bottlenecked by the speed of the output device and the transfer rate drops to the rate at which it can accept the data.
If you want faster transfer rates, you'll need to buy a faster USB flash drive. Be aware that USB 2.0 limits the maximum transfer rate to about 35-40MB/sec - to go beyond that you need a USB 3.0 flash drive and a port to plug it into.
I believe that Explorer is smart enough to keep showing the progress dialogue box until the file has actually been completely written to the output device. That's because otherwise some people would just yank the flash drive out of the USB port as soon as the progress box disappears, even though the system may still dumping data to it from RAM. So once the entire file has been read into RAM the transfer rate drops to the speed of the output device until the transfer completes.
It's a fluke of how windows measures data transfer.
Regarding Hard drives.
It's a somewhat close measurement.
The thing about windows is that it also takes the amount of files into account in the transfer.
Try copying 10 or more very large files. You will see that the speed and "estimated time remaining"will bounce around after each file is transferred, while it seemingly hangs while it is in the middle of each file.