Running High Sierra. I used a Seagate external hard drive and a Verbatim hard drive for Time Machine backup. Suddenly they are both not recognised by the iMac. The power lights are on and the disks are spinning. I have tried direct connection and via a usb hub. I have tried different cables. I have another Seagate hard drive which works fine in both the hub and direct connection and with all the cables. How do I wake up the 2 drives? Disk Utility does not recognise them either.