@ AndyE
If by your comment "When I go to Avasts website it says it is on my computer.", you are going to your my.avast.com account ?
Then the old installation (device) will still be shown as the program wasn't uninstalled as such. You may also have to manually delete the device from your my.avast.com account.
Then you can link the new HDD installation of avast to your my.avast.com account and also assign/use your license to that installation.