Time machine reports "already in use"

This is an old issue which has been re-introduced in Sierra.

Spotlight doesn't automatically ignore Time Machine volumes on third party hosts.

• Restart system and start time machine backup
• Wait until time machine volume is mounted in `/Volumes`
• Go to Spotlight privacy settings and add the time machine volume to ignore list

This should resolve it.

Did you open Time Machine Preferences and select the drive again? Or remove the drive, then, select the drive?

Backup is done on a WD My Cloud ...

Unfortunately, that is not a supported Time Machine configuration. To learn how to use Time Machine read Use Time Machine to back up or restore your Mac.

Despite their manufacturers' insistence to the contrary, there are no non-Apple NAS devices that will reliably work with Time Machine. If you want to use your NAS device to back up your Mac's contents, then use a backup strategy other than Time Machine.

The exhaustive list of devices supported by Time Machine are as follows:

• External drives directly connected to the USB, FireWire, or Thunderbolt ports on your Mac
• The built-in drive of an AirPort Time Capsule
• A USB drive connected to the USB port of a Time Capsule or AirPort Extreme 802.11AC base station
• A networked volume served by OS X Server using Apple File Protocol (AFP)

That is all.

Use whatever backup device you want, but you should be aware that this site is full of reports of misery from hapless individuals who had been using third party NAS devices for Time Machine backups, only to find that they were incomplete, corrupted, or useless in the dire circumstances in which they were required. Apple won't care if you lose your data while using a Time Machine configuration specifically excluded from their technical support documents.

Backup disks you can use with Time Machine - Apple Support

Have a nice day.

You're welcome!

This configuration now has worked for over 2 years ...

As long as you fail to follow Apple's published Time Machine instructions, it will never work. It may only appear to work, perhaps for a very long time.

Having now learned that your installation is not supported by Apple, your choice is to implement a supported configuration, or go without Time Machine.

Thanks for this! Solved my issue.