If a disk that is being emulated shows as ‘unmountable’ before a rebuild it will show the same after completing the rebuild as the rebuild process is simply making the physical disk correspond to the emulated one. If you have not previously successfully formatted the drive this will still be required. Note, however, that a format wipes any existing data on a drive so should not be used if a drive that was previously fine suddenly goes unmountable (in such a case a file system repair is the way forward).
the diagnostics that you posted showed you were getting continual resets on the drive which is probably why the format never completed! Such resets are typically cabling related (although other causes are possible), so hopefully the change you made has rectified the issue for this drive.
you should probably post new diagnostics now you have made a change so any feedback can be based on the current setup..