So, the OS X Yosemite installer checks your hard drive before installing. Makes sense, right? You don’t want something wrong with the drive leaving you with a broken installation. Now, you’d think this would be the first thing the installer does- while most problems can’t be fixed while the drive is mounted, they can be checked for and the installer would refuse to proceed until the errors are fixed.
Except… the Yosemite installer does not do this check until after the point of no return. If your drive fails the check, as mine did, the installation will fail and your original OS will be inaccessible. I had no opportunity to even try to repair the disk to allow the upgrade to proceed.
Now, thankfully, at the point in the installation where this check is done, the Yosemite installer is booting into a minimal OS X environment. Terminal and Disk Utility were both there, and the downloaded installer for the upgrade was still present. This did allow me to create a bootable USB installer to attempt to repair the disk. Unfortunately, the problems with the disk were too severe to be repaired. It reformatted fine, so it appears to have just been some data corruption rather than a failing hard drive, still, it would have been nice to find this at a stage where I could shove all my important data out over the network as I didn’t have the money at the time to run out and get blank DVDs, as I’ve just moved and moving is expensive as fuck(at least when it’s your first apartment you are moving to).
Yosemite isn’t bad now that I’ve got it installed. I’m not sure how much is due to improvements in Yosemite over Snow Leopard, or the elimination of accumulated cruft from SL or the Leopard install that was installed over, but this thing feels a good bit snappier. I just wish Apple had made the sensible decision to run the disk verification *first*, leaving me with a full OS X install to bring to bear on the problem, or at least a functioning system I could use and only run the upgrade again once I had the media or network connectivity to easily back up some data.