Imac 27" can't see camera with USB extender

[hedho]

For the first time I've taken my iMac27"out on a job with the same
software I run on my iMac24" & MBP 17".

My Canon 1ds Mk3 & 5D2 both run using the tethered system in
Lightroom 4.2 with no problems, using a 5m active USB extender.
They work all day long every day. But the 24" is running a bit slow with LR4.2, that's a WHOLE other issue (thanks Adobe).

So my problem is that, with the iMac27, the 1dsmk3 is not recognised by
the computer or LR4 when using the cable extension. BUT, if I use
the standard Canon length cord (approx 2 ft) it works fine.
So probably some power issue with the USB port?

I tried it in every port, and the same result, except the port closest to the headphone jack made the screen/computer freeze, when the camera was plugged in.

I also did a computer restart , Cmd-Opt-P-R to reset the PRAM etc.
Still no luck. I tried a different 5m active extender, and they same thing happened. I even had an error code come up on my camera "ERR70"
which can relate to a whole lot of image issues.

Can anyone let me know what I can do to test why the USB ports may have some confliction with the USB extension?

Thanks
Geoff (Auckland , NZ)