Restart the mac then connect the disk or connect them to another mac, if they are seen by diskutility select the "FirstAid" button; if they are seen but greyed, select "Mount" then select select the FirstAid button.
I was using the same cable on all my external drives which appeared to have been no well secured. All 3 drives are now not detected on desktop or disk utility despite lighting up and vibrating.
2 replies
Mac no longer detecting external drives after accidentally removing without ejection.