Experienced an issue with a WD MyCloud drive & Mac OS 10.12 Sierra backing up to the drive via Time Machine. The backup failed with the error “Resource temporarily unavailable”. I rebooted the MyCloud drive and the same issue occured. I noticed there was a firmware update 04.04.05-101. Applied this and it fixed the issue.
Here are the release notes to the firmware fix.
Firmware Version 04.04.04-101 (09/20/2016) Resolved Issues: Resolved issue where macOS-10.12 user is unable to create a new Time Machine backup
If your firmware version is using 2.x it has been fixed with 2.21.119 (09/20/2016)
Remember to update to the correct firmware depending if you are using 2.x or 04.x
You can download the firmware here – http://support.wdc.com/