25 MBps transfer between two external usb 3.0?

totalamateur

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Jun 27, 2015
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I left for work today after starting a copy of 1.27 terabytes of data between two USB 3.0 hard drives - a LaCie and a Seagate. They are both connected to USB 3.0 ports in a 2015 iMac (only usb 3.0 ports) they are both formatted to Mac OS Extended (Journaled). I'll admit I was lazy (as all of the data is very recently created and backed up) and just used the finder's copy routine, so I'm calculating this speed based on the amount it has gotten done now and the time its taken: 971GB in roughly 10 hours, which means the transfer rate is between 25 and 30 MBps. Is this what I should be expecting, or am I missing some very obvious thing which explains why my transfer is taking so long?

Note: The Seagate is brand new out of the box, and the LaCie is only a month old.
 
I've noticed when both of the devices are on the same USB, the available bandwidth is cut in half, as if USB is incapable of simultaneously doing RD and WR. Then just because ur on a fast highway doesn't mean all cars in it are Ferraris, really don't know, they can stuff a slow HD mechanism in a USB3 case all day.