I have this problem for a while but never reported, i'm on Manjaro Gnome 19 but it also affected me on 18. I tried to copy a 2.8 GB file to a USB Drive (NTFS partition), the progress bar on gnome moves quicl, in a matter of 25 seconds it's done but when i try to unmount the usb disk the system says it is still writing data... and it stays this way for 10min !!! The usb drive isn't slow, it is pretty fast on Windows.
I search the web and find lots os reports about the same problem on several linux distributions, but strangely no one has a solution. Anyone can help me ?