Mysterious problem with my Mac

I have an 11″ MacBook Air that I transport between home and office each day. In both locations I connect MBA to a 27″ Thunderbolt Display. And connected to each of those Cinema Displays are two 2.5 inch hard drives—a time machine drive, and a bootable backup drive.

Both the time machine and bootable backup drives at the office are encrypted, and the encryption passwords are located in my keychain. So when I connect the MacBook Air, they automatically mount and become ready for use.

The problem

From time to time, while using the MBA, I’ll completely lose access to application interfaces, making the computer all but unusable. I can thankfully still restart the computer by clicking on the Apple menu and choosing restart. If any application, however, requires anything other than mouse interaction to quit, then I’m stuck and have to force-restart the computer.

Although it’s not the only time this happens, the action that most reliably seems to triggers the problem is the mounting of a DMG volume that would need to prompt the user (like when I’m installing an application, or when an app is performing a self-update).

The only other clues I have are the following:

  1. When the computer is shutting down and the final gray screen appears, I’ll often see a dialog box asking me to enter the Time Machine disk password.

  2. Other times, I’ll see a dialog asking me to unlock the back-up drive (and I usually see this dialog when first accessing my account). If I enter the password in this dialog, it’ll simply shake at me. Canceling is the only way past.

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In both cases, the drive (Time Machine or Bootable Backup) were already mounted.

What I suspect is happening

I think the problem is due to some timing issues related to how I connect my computer.

I normally first connect the Thunderbolt display’s power to the MBA, and then connect the Thunderbolt cable. I then wake the computer using the Apple bluetooth keyboard. To disconnect, I simply dismount all drives, and pull the Thunderbolt cable out (putting the machine to sleep).

I’m guessing that at some point between connecting the Thunderbolt cable and actually unlocking the screen and getting into my account, the OS tries to mount these volumes and prompts for a password. But then my account is accessed, and via the keychain the volumes are unlocked—but these original prompts are somehow staying alive, and later interrupt the system when other volume-related prompts are delivered (like when a DMG volume mounts).

Also to note, when I happen to remember to open the MBA and unlock the screen before connecting the Thunderbolt display (and then re-closing to use only the 27″ display), I’ve not noticed this problem happening.

This is one of those problems that I imagine is so obscure, that Apple would never get around to fixing it (if they even know about it.) So I’m hoping by posting this to my blog, perhaps some others experiencing it will find their way here via Google, and leave a comment.

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