I haven’t tried this method but I suggest you look into FXP, which is supported on the EX2. It’s basically a server-to-server direct ftp without going the circuitous route of going via your computer.
I would say if you’re getting 10 MBps read speed then that’s awfully slow…not sure though what’s the culprit here as there are too many variables…it could be the other NAS is slow, could be RAID config on your EX2 that I’ve heard slows writes down (I don’t use RAID on my EX2, I use JBOD so I don’t know for sure). Also, if the either of NAS has services like media sharing on, I’d turn that off during the duration of file transfer.
I remember seeing some instructions in the EX2’s manual about making a way to schedule backups between EX2 and another NAS…that process may or may not apply in your situation but you should check out the EX2’s manual for that info.
Copying 4TB of data will take a long time regardless but you should be getting better speed than what you are reporting. Not sure what’s the bottleneck here.
Of course, there is also the tedious option to copy the data off your NAS in chunks to your local computer first and the computer-to-EX2…you may not have 4TB space available on your computer hard drive…I wonder if that could speed up the copy process. Of course, this would be a manual 2-step process but I wonder if data transfer would speed up this way. You could see what speeds you get for a small but sizable chunk of data (like 10-20 gigs) and then extrapolate the numbers to 4TB to see if it would be better. BTW, I suggest you try this using ftp (i.e. ftp files over from NAS1 to PC and then ftp from PC to NAS2)…my suspicion is you will see better transfer speeds…of course it’s still a pain in the butt method since you’ll have to do it manually and perhaps even in batches but if you have a large external (or internal) hard drive then you can minimize the number of manual fetches and subsequent puts. I suspected this might give better transfer rates…and just now I saw this post which suggests the same method to yield a little faster rate -> http://community.wd.com/t5/WD-My-Cloud/Slow-Transfer-Speed-over-USB/m-p/687003#M8322