apple imac 2008 BootCamp
To paraphrase your question:

You ran Boot Camp Assistant on a 2011 iMac to create a Windows 7 install disk on a USB drive using an existing Windows 7 install CD.

This worked when installing Windows 7 into the Boot Camp partition on that iMac but it is not recognised when attempting to do the same on an older iMac.

Is that right?

If so your problem may well be that some of the drivers required by the older Mac are not on the USB drive. Try creating the install USB using Boot Camp on the older Mac and it should work fine.