If you have experienced this issue since upgrading your Mac to OSX 10.6, here's the skinny on what's going on and what you can do about it. Basically, this occurs when attempting to access external drives (ie, USB, Firewire, iDisk, etc). Symptoms of this error include a missing menubar (unless an application is active and in focus) and no "active" indicator beneath Finder in the Dock. This particular error can prove to be extremely annoying since the majority of Mac GUI applications incorporate the Finder in the UX (ie, Open and Save dialogs). Luckily, it's not difficult to recover from, just depends on how you'd like to.

If you have running processes that you don't want to stop by rebooting your machine, open Terminal and issue this command:

killall Finder

press the Return key and Finder will be relaunched for you without a computer restart. If you don't mind rebooting, I'd suggest issuing the restart command from Terminal vs. using the Apple menu as the latter is prone to be blocked by the current issue. To restart from Terminal, issue the following command:

sudo shutdown -r now

You can replace the -r flag with -h to shutdown instead of reboot. Once restarted, your computer should be back to normal. There are also other options ranging from unmounting the disks, rebuild LaunchServices, applying/reapplying the 10.6.1 combo update and ultimately reinstalling the OS but I believe these two to be the fastest to getting your machine back up and running…