If you are running any 6.4 or later release, click 'Check for Updates' on the Tools/Update OS page.
If you are running a pre-6.4 release, click 'Check for Updates' on the Plugins page.
If the above doesn't work, navigate to Plugins/Install Plugin, select/copy/paste this plugin URL and click Install:
Refer also to @ljm42 excellent 6.4 Update Notes which are helpful especially if you are upg
This should have been posted on the general support forum since it's not v6.6.6 related, disks look fine, looks more like a connection issue, you're also using IDE mode which can cause problems, change the controller to AHCI in the bios, check/replace cables and rebuild the disabled disk.