I encountered a really strange issue with a Mac mini functioning as a small workgroup file server. The mini had dropped off the network and no-one could access it’s share points. I had the end user reboot it to see if that sorted it out and it didn’t so I went in and had a look at it in person.

When I got there, one of the first things I noticed is that there was no link light on the switch port that the mini was using. Strange… I tried different patch leads, both from the mini to the wall and from the patch panel to the switch. No go. I tried plugging the mini directly into the switch. Still no go. I looked in the Network prefpane, and sure enough, the mini was saying that there was no link for en0.

I tested the cables and switch port with my laptop and they all worked perfectly.

At this stage I was beginning to suspect a hardware issue. I can’t recall the last time I saw a failed Ethernet port on a mini, but anything is possible. I checked Time Machine to make sure there was a recent backup before attempting anything else and there were some errors in Time Machine as well.

I reset the Time Machine backups and Time Machine kept coming up with an error, every time I tried to tell it to back up to the external hard drive — Time Machine could not be configured. The built-in network interface could not be found.

Yes, I know that the network interface can’t be found, that’s why I want to back up! I deleted the Ethernet interface and tried again – same error message. I re-added it to the system preferences and still the same error message. I thought that it was strange that you couldn’t run a Time Machine backup if your machine wasn’t connected to a network, so it was time for some googling.

Apple have a knowledge base article Time Machine: Troubleshooting backup issues that was exactly what I was looking for.

Red “i” message: “Time Machine could not be configured. The built-in network interface could not be found”

From the Finder’s Go menu go to this location:
/Library/Preferences/SystemConfiguration/

1. Copy the file “NetworkInterfaces.plist” to the desktop (to make a precautionary backup).
2. Delete the file “NetworkInterfaces.plist”. Enter administrator name and password if prompted.

Restart your Mac.

3. Once restarted, reconfigure your the Network and Time Machine preference panes in System Preferences.
4. If the issue persists, contact an Apple Authorized Service Provider to have your Mac’s network ports checked.

Sure enough, deleting /Library/Preferences/SystemConfiguration/NetworkInterfaces.plist, rebooting and then setting the preferences in System Preferences fixed everything as good as new.