|Craig, so you need to image/clone the operating system to one of your other drives.|
Bear in mind I am no expert. But this is what I would do. If there is nothing you want on the 500, put it back in the pc, set this drive to slave.
Then, go to the hard drive manufacturer & find a Utility to clone/transfer to a new drive, or use Ghost.
You will need to read up about this. Ghost/image, to the 500, then remove the 1 terra & fit into an enclosure.
Read this page, & look at the links
Discwizard copies the os over to the other drive, the link has instructions for this.
Other drive manufacturers have similar instructions
Before posting try google. Backup. Use anti virus software.