Weird OSX Lion bug :mini DVI prevents boot/reboot

Today I just could not connect to my MacMini which was displaying a screensaver when I turned the screen off this morning. Usually it works fine when I come back home and turn it back on, but this time it only showed up a black screen, meaning the screen was connected and it was resuming from sleep.

So let’s investigate this

Before trying anything else, back to the basis : it replies to ping - good, so OSX hasn’t crashed.

ssh and Screen Sharing (VNC) also work - so I click on Apple/Retart, all the windows clause but nothing else happens. The VNC connection is not closed.

Weird. I try the same by ssh : sudo then reboot - but reboot also fails with :

/ is busy updating; waiting for lock

Weird. Let’s check /var/log/system.log:

May 23 18:30:57 macmini com.apple.kextcache[41843]: Lock acquired; proceeding.

May 23 18:30:57 macmini com.apple.kextcache[41843]: /: helper partitions appear up to date.

May 23 18:30:59 macmini AppleFileServer[41769]: _Assert: /SourceCache/afpserver/afpserver-585.2/afpserver/../afpserver/SharePointSyncTask.cpp, 215 (4294967253)

May 23 18:31:25: —- last message repeated 1 time —-

May 23 18:31:25 macmini com.apple.backupd[41859]: Starting standard backup

May 23 18:31:25 macmini com.apple.backupd[41859]: Error -35 while resolving alias to backup target

May 23 18:31:35 macmini com.apple.backupd[41859]: Backup failed with error: 19

Weird. After some googling, I see hackintosh users have the same problem with a wrong ACPI VGA kext, but maybe backupd is causing the problem? A killall -9 backupd later :

May 23 18:31:52 macmini com.apple.launchd[1] (com.apple.backupd[41859]): Exited: Killed: 9

Let’s try again to reboot

May 23 18:32:06 macmini reboot[41874]: rebooted by guylhem

May 23 18:32:06 macmini reboot[41874]: BOOT_TIME: 1337812326 786930

Yey, it works ! Maybe it is completely unrelated to ACPI/VGA kext. But then I get nothing but the same black screen at reboot. WTF? Now I can’t even ssh!!

Time for some deep thinking - so I remember about anything I may have done this morning - but I only did one unusual thing : I cleaned my desk. How could it be related??
Some deep thinking later - what if it was due to a loose cable? After unplugging/replugging the Mini DVI cable, everything works again !!!

Morale of the story: in OSX there can be weird bugs too. A misconnected mini DVI cable *can* apparently cause backupd to fail, while still powering the display to wake it up from suspend, but causing reboot problems and preventing a normal boot due to the kext - just like on a hackintosh. 
Hardware bug of software bug, whatever it may be, that’s still strange. Hopefully someone with the same weird problem will find this solution thanks to google.
Big kudos to the hackintosh forum - they can be very helpful, even for vanilla macs runnning the vanilla latest stable version of OSX.

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