iMac screen

Hi Guys,

My dilemma is, I have an older (mid 2010) 27" iMac and a brand new Macbook Pro retina 15" laptop.

I would like to use the iMac larger display connected to the Macbook on occasions I wish to do some graphics work.

The iMac has a Mini Display port and the Macbook has a Thundercloud port.

I seem to recall that I can connect the two together using a Thundercloud double ended cable to connect the two together which will work in that the display will travel from the Macbook to the iMac (but not in the other direction?).

I went into the Apple Store in Leeds today and two different reps in there said that it could not be done as the iMac was too old to handle it.

Does anybody have a different view on this, or will I have to go to a greater expense to achieve my desire?

Thanks my friends

Dennis