Something tells me you have your cables or pin configurations mixed up.
First, take a look at the graphic images in the upper left corner of this page:
http://pinouts.ru/Slots/ieee1394_pinout.shtml
The most commonly used firewire 400 connector is 6 pins, with one end "flat" and the other end "rounded".
The new firewire 800 connector is 9 pins, rectangular (NO rounded edges), almost-but-not-quite "square".
You wrote:
"I'm trying to move files etc to the new from the old. I have everything backed up on a Maxtor external HD with Firewire 800."
I'm going to _guess_ that your old Mac has firewire 400, is this correct?
In that case, it has firewire 400 ports with one end flat and the other end rounded.
If the external drive is in fact "firewire 800", it has either:
1. The new firewire 800 port (9 pin rectangular), or,
2. BOTH firewire 800 and firewire 400 ports (for "backward compatibility")
In that case, the cable you have NOW and are trying to use is either:
1. A firewire 800-to-400 "conversion cable" (it has a 9-pin connector on one end and a 6-pin connector on the other end), or,
2. An older firewire 400 cable (with 6-pins on BOTH ends), that is plugged into the firewire 400 port on the external drive.
If the external drive you have is an older firewire 400 drive (again, you must check the ports to determine this), you need a firewire 400-to-800 (6-pin to 9-pin) conversion cable to connect it to the new iMac.
If the external drive is in fact a newer firewire 800-capable drive, you need a firewire 800 cable with the 9-pin rectangular connectors on BOTH ends.
The best source for cables I've found is monoprice.com.