Imagine you have to switch the mainboard of your Apple computer. More specific: the network card or even more specific the MAC address of your network card. Then your exisiting TimeMachine backup will no longer be recognized as valid. This is because of the fact that the Backup folder on the TimeMachine Volume is tagged with the MAC address of your computer. That should prevent you from accidently using TimeMachine backups from different computers. But will also bring you trouble if your MAC address changes for some reason (as in case of mainboard switch during maintenance).
Unfortunately there is no obvious way to tell TimeMachine to use an pre exisiting Backup. TimeMachine would simply start a new backup if you try to continously use the old Volume. The old Backup folder may be even moved to the Trash on that Volume. Obviously this is not what you want.
But when you try to move the original Backup out of Trash, you’ll notice that even the super user “root” doesn’t have the privileges to do that.
But don’t worry. You’re not all lost. What is causing you the trouble is the fact that the MAC address is somewhere tagged on the Backup folder and that Apple additionally uses ACLs (Access Control Lists) on the backup volume. These are basically some additional security settings which should prevent unauthorized or unwanted changes.
So the easiest way to get the Backup working again is to disable the ACLs on the volume, change the MAC address tag of the Backup folder and the re-enable the ACLs on the volume.
This can be done in the Terminal.app as superuser:
cd /Volumes/TimeMachineBackup fsaclctl -p /Volumes/TimeMachineBackup -d xattr -w com.apple.backupd.BackupMachineAddress 00:16:cb:90:62:0d `hostname` fsaclctl -p /Volumes/TimeMachineBackup -e
After that your original Backup should be recognized by TimeMachine.
This tutorial was found here: CyberHQ NL