I own two external Western Digital MyBook USB hard drives, which I have used with my old iMac (died recently (sigh) ) without any problem. The hard drives have been connected on separate iMac ports using USB2.0 and I have turned on WD Security.
Now I have a new Mac mini and I have migrated (with the Assistant) my old settings and everything from my old internal drive to the new Mac mini internal drive. Since the Western Digital hard drives support USB3.0, I have connected both drives to the USB3.0 ports.
Everything works fine until the mac goes into sleep mode and the drives spin down. After waking it up, the drives start to spin up but I am not longer able to view/access the content of the external drives. It seems that Finder does not wait long enough and does not reattempt to access the drives.
What I have tried:
System Control->Energy Settings: Do not deactivate hard disks
WD Utility: Deactivate Sleep Timer
Maybe MacOS has migrated old drive configurations from the old internal disk which causes the problem but I don’t know how delete those configurations.