Cannot mount two hard drives

Having a crisis so any help would be appreciated

My 1tb hard drive is failing (can be read but does not boot) so have bought a new one to transfer everything over to, whilst using an old 500gb in my mac (so am plugging in both 1tb via usb)

When I plug one in the other immediately force ejects and vice versa, both have different names and am at a loss what I need to do, can't find anything by googling either

thanks

simsaladimbamba said:

Are those drives 2.5" drives without power supply or 3.5" with a power supply?

I presume, you use the 13" MacBook Pro, listed in your signature as Mac Pro?

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Sorry, yes its a macbook pro

They're 2.5" drives and am using two external cases that power via usb

Actually I'm seeing what the issue may be, not enough power from the pro?

simsaladimbamba said:

Is your MacBook Pro connected to a power supply?
Do you have a powered USB hub?

I only have a 17" MBP and it is able to power at least two 2.5" HDDs directly connected to two of the three USB ports.

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I'm connected to a power supply but am only using a standard hub

http://www.pcworld.co.uk/gbuk/advent-hb112-4-port-usb-2-0-hub-13212939-pdt.html

It's ok, have sussed it one usb straight into the mac then the other into the port, so was a power issue (it's a cheap ebay case and threw me with it having to three usb cables, 1 for drive two for connecting to laptop)

Thanks for your help