After reformatting the new drive, and cloning the OS to it from my working Imac, I pulled apart the old Imac (24 inch aluminum 2.8ghz) and replaced the drive. Once I got it put back together, I put the old drive between a couple gel ice packs in a box on my desk and was able to import the user accounts from the old drive to the new drive via Migration Assistant. All the various files have been copied back from the external drives, and the machine is purring with its new 1.5tb seagate drive ( I read the 2tb drives are more prone to failure so I opted for a proven-and much less expensive 1.5tb drive). I also upgraded from 2 gb to 4gb RAM while I was at it.
Thanks for all the help!