[pmontra]

I also noticed that on Gnome. I never investigated the implementation details of that progress bar but my gut feelings is that the file is read from (or written to) the buffer cache quickly and the progress bar goes near to 100%, then stays there until the last writes succeed and actually write to the USB stick. Then the eject button sometimes need extra time to finish the sync and tells me to wait a little. I always remove the stick when it tells me it's safe to do it.

[loeg]

Your intuition is correct. The writes are quickly buffered to RAM and then fsync or close writes them out to the slo media. The (naive) progress bar probably only tracks progress buffering the writes — it's the simplest way to track progress, if inaccurate.

[spatular]

Yes. You can also run "watch cat /proc/meminfo" in console and watch "Dirty" and "Writeback" fields to see dynamics (and how long you have to wait).