Originally posted by Gene Clean
Why is verifying and repairing your HD a must before updating?
Because little errors pile up. Sometimes they are rectified by the simple task of checking the drive, and Disk Utility doesn't let you know. But, if your drive does have a real problem, no matter how minor, when you install and update, that error can corrupt part of the install. Sometimes it's subtle, sometimes it's major.
An example. Right before 10.4.5 came out, my machine seemed a bit sluggesh. Just by a tiny amount. I check my drives once a week, so I didn't think much of it.
When the update came out, I verified my drive, but Disk Utility said that it couldn't finish because the wrapper was corrupted (I don't recall the exact wording). It stated this in BOLD red type. Back up the drive etc.
I restarted from another drive that has all of my tools on it (and a CLEAN OS), and tried Disk Utility again, but this time set to fix the drive. Same notice.
I went to the current version of DiskWarrior (3.0.3) and tried again. It failed to replace the directory!
After turning the machine off and on a few times, waiting for the RAM to drain down each time, I tried DW again. This time it worked. I tried Disk Utility again. Fine. I then installed the update.
Many disk problems do not show any trouble during daily use, but if you don't check before an OS update (or upgrade), you are just asking for trouble.
Anytime you install anything at all on a drive with any kind of directory trouble, you can install over some other bits of code, causing anything to happen. Or the code could have the wrong information, information the update needs to know.
And, you also have to remember to unmount all firewire drives, turn them off, and unplug them from the machine before doing any OS upgrade as well. Then after all is finished, you can turn them on, and plug them in again, before turning the machine back on.