Time Machine backup failed. Why?

[Sarcany]

space-time wrote:
[quote=Sarcany]
Happens when there's a network-interruption during a backup and often corrupts the target image.

Yes, reboot the NAS to clear the "busy" flag on the image.

Follow Up Question.

Is the target image still corrupted? Should I worry about that?
If your NAS permits you to mount the backup vol over the LAN then you can use the Disk Utility to verify the image file.

space-time wrote:
What happens if my MBP is backing up (via WiFi) to NAS and I close the lid, MBP goes to sleep. Does NAS close the target image? or it stays open until I wake up the MBP?

It's likely to be marked busy until the MBP is woken and if network operations are resumed then the Mac should gracefully unmount the image. But lots of stuff could mess that up.

Network Time Machine backups are convenient, but fragile and less useful than backing up to an external hard drive.