Stephen Hackett today updated a great, easy-to-follow guide for setting up a Time Machine server on your network. This is something I have been meaning to do for about a year and I figured a Friday evening before a long weekend would be a superb time to make it happen. After all, I already had a Mac to use as a server — my MacBook Air I upgraded from last year — and a hard drive. And Hackett describes it as “easy”.
How hard could it be?
Well, the first series of steps in Hackett’s guide took me fifteen minutes, and ten of those minutes were spent trying to find a Thunderbolt cable. Then I got to the part in the guide where it says I should be able to authenticate and mount the drive, and I hit a wall: I could not move past the user name and password dialog. It was not that my password was being interpreted as though it was incorrect — that comes later — but that it would accept it and then show the dialog again. I could not even mount the external drive in Finder, and sometimes it struggled to mount any drive on the host MacBook Air. I kept seeing errors like “The operation can’t be completed because the original item for ‘Remote Backup’ can’t be found”, and “There was a problem connecting to the server ‘Remote Backup’. You do not have permission to access this server.”
At some point, I also managed to lose access to my MacBook Air through the Screen Sharing app on my MacBook Pro. I would type my user name and password and it would reject it, as though I got either one wrong. But if I launched Screen Sharing from the network-mounted MacBook Air in Finder, it worked fine.
Hours later, I found the solution. System370 on Reddit pointed out in a months-old thread that smbd needs to be granted Full Disk Access permissions in System Preferences on the host Mac. That is the SMB protocol daemon; SMB is the file sharing protocol used to mount the drive on a remote Mac. I enabled Full Disk Access for the daemon, completed Time Machine setup on my MacBook Pro, and it is now creating a Time Machine backup remotely.
I mean absolutely no criticism of Hackett or this guide. In fact, I am grateful for the reminder to set this thing up — finally. But none of the error messages I saw on either machine nor any of Apple’s support articles mention this simple yet critical step.
So, here it is again: if you are enabling File Sharing for a remote disk and something is not working, skip all the troubleshooting you find elsewhere on the web. First, ensure smbd has Full Disk Access, under System Preferences (or System Settings), Security & Privacy.
I hope this keyword-filled post saves others some troublesome troubleshooting, and that Apple will reconsider its strategy of erroring in silence or with irrelevant messages. This is apparently a known problem because, yes, my host MacBook Air is stuck on Catalina.