When a new OS X update is released, I'm often the one who digs into it for Macworld, attempting to see what's new and different. Such was the case with the recent OS X 10.5.7 update. Before diving into such a project, though, I like to make sure that the update doesn't cause any catastrophic issues, so I upgrade at least one machine in the house. I don't, however, upgrade all of them, because it's very useful to have a previous-version machine to compare with the just-updated machine while investigating the changes.
So after upgrading the Mac Pro and the MacBook Pro to 10.5.7, I used both machines for a while to make sure everything appeared to be working. Once I was confident there weren't any major issues, I started working on the article, using our 12-inch PowerBook G4 as the 10.5.6 comparison machine.
After the article went online, I updated the PowerBook, and that went well. That left just one machine, our year-old iMac. This machine is used mainly by the kids for fun and games, and for our household bills, inventory, and other such tasks. It is, by far, the most "stock" machine of the four in our house, not being subject to Mac OS X Hints work, nor sporting additional hard drives and/or partitions for Boot Camp and other versions of OS X. It was, in short, the machine I expected the least trouble with.
Of course, that meant that absolutely nothing went right when I tried to update it. First, the update wouldn't download, spitting out an error message stating that the digital signature on the package wasn't correct. Having just installed it on three additional computers, I was pretty certain that wasn't true, and I wasn't the only one seeing the message.
I launched the installer and then went away to play with our kids for a bit. I heard the iMac reboot, which was expected. When I wandered back over to see what it was doing, however, all I saw was a blue screen with the cursor in the middle of it. Browsing some of the comments on my 10.5.7 thread, this apparently happened to a few other people as well.
Commenters in the thread suggested just waiting, so I tried that...30 minutes later, though, I was still looking at a blue screen. Other suggestions included rebooting a couple of times, so I tried that via the power button (as the keyboard didn't seem to be working). Still no go--I'd hear the boot chime, and see the Apple logo briefly, but then be back to the blue screen. Holding down Command-S at startup (to watch the boot process, to see if I could see where it got stuck) didn't work either, nor did holding C to boot from the CD-ROM. It was like the boot process wasn't even starting.
At this point, I was beginning to think I was in for a reinstall from the iMac's recovery disks, though I wasn't sure how I'd do that--there was a music CD in the iMac's slot, and no amount of trickery would get it to eject.
Finally, I remembered the old standby, FireWire Target Disk Mode (FWTDM for short). For those not familiar with it, this special boot mode turns any FireWire-equipped Mac into a FireWire hard disk when you connect the FWTDM Mac to another Mac. You boot into FWTDM by holding down the 'T' key at startup; you'll soon see a massive FireWire icon filling your computer's display, indicating the machine is ready to connect to another Mac.
After starting the iMac in FWTDM, I moved the MacBook Pro over to the iMac and connected it to the iMac via a FireWire cable. The iMac's hard drive then showed up on the MacBook Pro's desktop, as did the music CD in the iMac's CD drive--so FWTDM works with more than just the hard drives in the target Mac.
The first thing I did was eject the music CD, just in case I did have to reinstall OS X from the system restore discs. Next, I found (on the iMac's drive) the Combo Update that I'd previously downloaded, and double-clicked it. The installer started, and offered up the iMac's disk as a valid destination for the upgrade. I selected the iMac's drive, and let the installer do its thing.
This time, everything worked perfectly--the installer finished, the iMac installed the update and rebooted, and the normal login screen showed up shortly thereafter. I have no idea what went wrong the first time, but I'm very thankful FWTDM was there to save me the hassle of reinstalling from scratch. If you've got more than one Mac and both have FireWire ports, do not overlook the power of FWTDM for recovering from seemingly impossible situations.
This story, "FireWire Saves an IMac From a Failed 10.5.7 Upgrade" was originally published by Macworld.