Not finding this question answered anywhere: Recently upgraded to OS10.12 on my iMac. I have 2 LaCie external hard drives attached via USB.
As long as the hard drives are awake (spinning, blue light on the front is on)---they function fine---I can view the contents in one and the other which is devoted to my Time Machine back up is backing up hourly as programed.
However, after installing Sierra, I noticed the hard drive devoted to Time Machine back up would, after sleeping, not be available for back up. ("Time Machine failed") When I click on the icon of the external on my hard drive---it appears as being empty.
Attempting to mount/unmount the hard drive in Disk Utility creates the following error message: "Failed to eject (name of drive) because it is currently in use." Restarting the computer "rights" everything: both hard drives are spinning (blue light on), contents are there and available for edit/or back up.
I was able to reformat one of the hard drives using disk utility but this did not correct the problem.
This was not a problem prior to installing Sierra.
