Well, I have to say this thread piqued my curiosity! I am a new owner of a QNAP TS-219P. I formatted an external USB HD as Ext4 and setup the NAS for scheduled synchronization. The first one worked fine (5 hours for about 100GB of data). Seemed long, but it worked! The second scheduled synchronization failed - and that's all I can find out! There's no log or detailed error message. So, I emailed customer (since I am one) support and haven't had a response in 3 days. Since the schedule is for every 2 days, the second try failed as well. Again, that's all I know. I tried the forums and found other users with the same issue. It's not that it's slow, it actually doesn't work! Now that's a failure.
I've tried a manual backup and a manual synchronization, verified that the USB drive is accessible and readable. Still no success and no additional insight.
Unfortunately, I can't seem to get any help from QNAP in figuring this out. Of course, I'd like a solution, but no one has contacted me or anyone else in the forums that I can see! Even if just to post to the forum with some kind of response. Nothing so far. I see responses in other threads, but the threads with USB backup problems are uniquely silent. THAT is probably more aggravating than anything. Not even an acknowledgment that there might be a problem.
Unfortunately, the effect will be to tarnish the reputation of QNAP. One thing I've learned as a consultant: keep your clients informed and be responsive to their concerns.
And yes, I'm a new member to the forum. I do not have any affiliations with QNAP or their competitors AFAIK.