Can I watch the progress of a `sync` operation?
Looking at /proc/meminfo will show the Dirty number shrinking over time as all the data spools out; some of it may spill into Writeback as well. That will be a summary against all devices, but in the cases where one device on the system is much slower than the rest you'll usually end up where everything in that queue is related to it. You'll probably find the Dirty number large when you start and the sync finishes about the same time it approaches 0. Try this to get an interactive display:
watch -d grep -e Dirty: -e Writeback: /proc/meminfo
With regular disks I can normally ignore Writeback, but I'm not sure if it's involved more often in the USB transfer path. If it just bounces up and down without a clear trend to it, you can probably just look at the Dirty number.
You can look at the /sys/block/<device>/stat file for the appropriate device while you're syncing. The 9th column will indicate the number of in-flight requests on the device, which should go down to zero when the sync is done.
Don't know of a way to translate that to a number of bytes, but it should give you a rough idea of how much "stuff" is still pending.
See the stat.txt file in the kernel documentation for a bit more information. (There's also an inflight file in that directory on my system which looks like it could contain read and write in-flight requests, but I can't find docs for that.)
By using Greg's answer, you can simply have sync run in background while displaying the state of the Dirty block in memory.
To achieve this, simply run this command:
sync & watch -n 1 grep -e Dirty: /proc/meminfo
This will call sync in the background while executing watch in the front. When the sync command will have finished (around when the size of the Dirty block has reached 0), you will have an output that looks like this :
1] + 27260 done sync
This means that the command has finished and you can kill the watch command with Ctrl+C.