I'm working on an article for Macworld that requires installing and removing a number of programs on my Mac--programs that include kernel extensions, frameworks, etc. Since I prefer to keep my core OS X install relatively clean, I created a new 10.4.6 installation on a FireWire drive, and I've been using that for all the software testing.
To make things even easier on myself, I did this all on the PowerBook, so I could continue working on the G5 while the PowerBook was involved in the testing process. When I put the PowerBook to sleep, the FireWire drive stays powered up--since the FirewWire port gets power even when the PowerBook is sleeping. Since the drive makes a bit of noise, and leaving it powered up bothers me, I've taken to unplugging it when the PowerBook is sleeping.
This morning, I woke the PowerBook as usual...but completely forgot it was booted off the FireWire drive, which was peacefully resting next to the PowerBook, unplugged. Uh oh. As soon as I realized what I'd done, I was ready for instant death in OS X. But no such thing happened. Of course, nothing much else happened, either--mouse clicks seemed to be ignored, Command-Tab didn't work, etc. In short, the machine was effectively locked up, though I could move the cursor. This makes sense, given that the system was sitting there without any way to access its operating system.
Since I was sure I was in deep trouble at this point already, I did the only logical thing--I plugged in the FireWire drive and crossed my fingers. Amazingly, it just worked--even the mouse clicks I'd made were 'remembered' and all activated as soon as the drive came back online, and everything was fine from then on.
This may be old news to many of you, but I was pretty impressed that I didn't immediately kill my PowerBook when I woke it up without a boot drive attached.