Self note in case i forgot in the future:
RPI2+External powered USB hub with a 3Amp power adaptor.
Hard disks attached to external usb hub are all USB3 and HDDs are all powered by the external USB hub
The RPI2 is also powered by the external USB hub
Accidentally I plugged an USB3 to the USB ports on the RPI2 and the following appeared in dmesg
[ 1360.915902] usb 1-1-port2: over-current change
[ 1361.129815] usb 1-1.3: USB disconnect, device number 11
[ 1361.289713] usb 1-1.4: USB disconnect, device number 9
This means the RPI2 was not proving juice to power the USB3 disk. Changing back the USB disk to the external USB solved the problem.
However this led me to find this very interesting page via google:
Here basically by software we can up the 600ma drawn by the usb ports on the RPI B+ I have to try an see if this also works on RPI2.
Note that the use of this procedure will fry your RPI if done incorrectly.