USB 3 1TB HDD

Ultra 4 only features USB 2 ports - it should work, but no faster than a regular USB2.0 HDD

What speed should it copy files at usb 2 speed if attached to the nas directly..

i remember when i tried this a while back it was painfully slow!

Average USB2.0 read speeds are 20-25MBps - I've certainly seen this level of performance with my Ultra 4

Hmm, I connected the USB to the front of the ultra 4.

Copying near 1TB of films from the internal drive to the USB..I'm getting no more then 6MB/sec..

Does this mean there is fault somewhere?

Possibly. Note that you asked about *READ* speed originally - "to copy files to the NAS HDD" - write speed will be slower, but IIRC I saw ~15MBps in my original testing which is again not untypical for USB2

Other than that, my speed measurements were just via a standard Frontview backup job, though always with the rear ports (for no particular reason) and using EXT formatted disks