I have now had 3 disks from WD, 2 replacements under RMA where I can access the disk when first plugged in and powered up, I can successfully write/read from the disk but once the iMac goes to sleep I am unable to access the files and get read errors. I am unable to unmount the disk and have to plug the USB lead in again to get access.
I have refomatted, tried keeping the disk awake all with no success. I am running Sierra on a mid 2011 iMac. My older My book is working just fine. Has anyone any ideas