Having reconfigured the drives as required, they may now be mounted into the appropriate drive bays.
In fact, the best arrangement was to move the existing drive up a bay – so as to maintain maximum air circulation and to swap the old hard drive’s power supply socket with that of the CD-ROM drive.
Reattach the drive cage, check that no other cables connections have been disturbed. It’s probably better to leave the case cover off for now in case you need to troubleshoot the installation later.