OK Graham - sync did wait on Fedora for the data to finish flushing to disk - its an older Fedora and the sync command is version "sync (coreutils) 8.15". Both ClearOS 6 and 7 have newer versions so would expect them to execute in the same manner...

Monitored data transfer activity both by watching my gkrellm displays as well as field 9 in the output of "watch -n 2 cat /sys/block/sdc/sdc1/stat" run in a terminal. Field 9 counted down to '0', the gkrellm indicated transfer had finished and the sync command returned all at virtually the same time.

Incidentally - if using a device with partitions then the stat file to monitor would be in this format "/sys/block/sdc/sdc1/stat" - in this example partition 1 on device sdc...

So it should be safe to umount and unplug the USB device after the sync command returns - assuming no more write have been subsequently issued to the USB drive.