Over the weekend, my iMac locked up after waking up. I could not get the machine to do anything - dock would not come up, totally unresponsive. So, I manually shut the machine down, and then upon bootup, the system would not boot up. It took 10 minutes to get to a blue screen with the cursor, but nothing else happened thereafter. Fortunately, I had a month-old backup on an external USB drive. (I use Rsync to do backups, it's shareware that does the job and not much more, so I'm more or less waiting for Time Machine) By pressing the Option key during startup, I got the option of booting from the USB 2.0 drive (my understanding is that this is something that you can only do using an Intel Mac -- PPC Macs require a Firewire drive). Once I booted up, I found that the Macintosh HD could not mount. I tried the Disk Utility, but it gave me an error message saying that it could not be repaired. It could not even identify the errors on the drive just the message "Underlying task reported failure on exit" Since I have AppleCare, and found out that the TechTool Deluxe utility that comes with it has a directory repair utility, I decided to try that out. As expected, TechTool flagged problems with the directory. When I ran the repair utility, it informed me of "unusual" errors in the directory structure, and asked me if I wanted to replace the previous directory structure (with the requisite warning that I might lose my data). I decided to do the replacement. And when I reran TechTool, it still flagged errors. So, I went back to the Apple Disk Utility just to see if the drive could at least mount this time. Tried the disk repair routine again, and this time the utility could actually identify the problems in the disk structure. And this time, Disk Utility DID repair the drive directory. It could now mount, and the files were all readable. Tried rebooting, and now the iMac properly booted from the internal HD. All seems well, but the machine is now taking a long time to boot up (probably about twice as long as before), and it seems that things like the Dock don't appear quite as smoothly as before. Granted, I don't reboot very often, but is there anything else that I should check or utilities that I can run to make sure that my system is running optimally? I periodically use IceClean and XSlimmer for basic maintenance and disk cleanup, but am otherwise unfamiliar with what's out there (coming out from Windows, I'm used to having to run the usual assortment of defragmentation and Registry cleanup utilities). Otherwise though, I'm very impressed with how seamlessly my iMac could just boot up from a USB drive and retain normal functionality, while I try to revive the internal HD. Lot more hoops to jump through to get a Windows PC to do the same thing. Needless to say, I'm very sold on backing up to an external drive regularly.