I'd suggest that rather than tossing/selling the old drive, install it in the new computer and keep it for a few months. There are always things you forgot to move over to the new system, so it's handy to have them right there in the box. Copying over big files (e.g. SONAR projects) to the new drive will be much faster and more convenient.
Once you're absolutely sure you'll never need anything on that drive ever again, wait another month anyway. Then wipe it clean and use it for backups or sample libraries or videos of your kid's Christmas pageant.
BTW, this past week I installed SONAR on a new laptop. I'd tried to expedite the process by copying over the installation files and running them, but CCC wouldn't authorize the install and I came up in demo mode. I then uninstalled SONAR and let CCC do everything from scratch, and then all was well.