To swap the HDD, only steps 1-9, 12, 17-20 is needed. As you re-insert the HDD, leave a spudger under the edge to prop it up against the grille - much easier to screw it back to the grille. (My mac mini has no HDD bracket)
Should have read your comment first! Would have saved me an hour....
This connector has a small catch on the right. slipping a sharp object (eg: pin) between the connector and it holder will release the catch. The connector should now come off easily.