I have an SSD installed, and when I'm transferring to or from an external HDD via USB 3.0 I get speeds of upwards 100mb/s.
However if I'm transferring between 2 external HDDs both on 3.0 I get a staggering 10mbs. Is this uncommon?
The drives are the limiting factor, not the bus. What units are those? bits? bytes? If you are saying 10Mbps, then yes, it is fairly slow, so I would think that one or other drive is having problems or not connected well.
If it is 10MBps, then that is still pretty slow, but not abnormally so. Are the drives (particularly the destination drive) fragmented? If one or both drives have fragmentation, then the two heads have to move a lot more, causing a slow-down and 10MBps is not unreasonable in that situation.
If one of the external HDD is not SSD this is normal beacause the slower drive is creating the transfer speed limit.
Try transfering from one drive to another and then back. Is the speed "staggering 10mbs" both ways?
Otherwise if not already the case connect both devices on the motherboard USB 3.0 ports.